The number of calls to Britain’s ambulance services is at an all-time high during the Christmas and New Year party season.

Sunday, 1.07am: A woman is lying face-down in the middle of the road. We are half-way through a 12-hour night shift, and approaching a major intersection when we spot her in the beam of our headlights. At the same moment a man leaps in front of the ambulance, flagging us down with both arms. We screech to a halt.

Outside, the bitter night air slaps against our faces. The pavement is blocked by onlookers, their voices blurring into one another. Just metres away, cars continue to hurtle past, the blue flashing light of the ambulance reflecting against the wet tarmac.

Paramedic Mervyn Dupont and his crew-mate, emergency technician Emma Smith, have been on the job for six hours, and from the moment the first call came in, they haven't stopped. Since then, it has been one gruelling pick-up, high-speed ambulance ride, and brimming casualty ward after another. And there are still six more hours to go.

For ambulance workers across the country, the circumstances surrounding this latest call-out are all-too familiar. Having had several drinks too many, on her way home from the pub with friends, the twenty-something who now lays unconscious in the road had jumped on a child's push-scooter at the top of a hill. At the point where the road hits a busy intersection, she had been thrown head-first from the scooter.

Miraculously, there is no long-term damage; other than concussion, cuts and bruises, the woman is largely unscathed. Yet at this time of year, during the peak Christmas and New Year party season, with an influx of cases like this to cope with across the country, the prognosis for an already massively over-stretched NHS system is less cheerful.

the number of 999 calls to the London Ambulance Service is now at an all-time high – and as many as 12 per cent are alcohol-related. Since the introduction of new licensing laws, the figure has risen from 48,000 in 2004-05 to 60,000 in 2008-09. And this trend is reflected in most major British cities. In monetary terms alone, it costs the tax-payer an average of £223 for each call-out to a drunken episode such as this one in north London, but the drain on human resources is even more costly.

Once crew members Mervyn and Emma finally lift the concussed woman from the road into the ambulance, complete a physical examination, transport her to hospital, and fill out the necessary paperwork in order to file a report, more than an hour has passed. All the while, a stream of urgent requests for assistance pours through from the control room, and none of these can be answered. But the strain on services doesn't end once the emergency medics have done their bit.

When a patient is deposited in hospital, they then require the attention of staff nurses and doctors, and – whether they need further treatment or not – they will have to be placed in a bed. And as Emma Smith pronounces: "There just aren't enough to go around, and we can't magic them out of thin air."

At this time of year, when binge-drinking is at its peak, the London Ambulance Service is pioneering several alternative ways to deal with the excess pressure. At great cost, it has also been trialling "Booze Buses" in the West End – Transit-sized vans in which to treat intoxicated but not seriously ill patients. Weekend field units have also been erected in the City, specifically designed to accommodate inebriated office workers spilling out of Christmas parties. After just a couple of weeks in action, staff have already seen hundreds of patients.

But alcohol misuse is not the only reason resources are under increasing pressure. Medics are persistently drawn into cases that would be better treated by a GP or an NHS Direct advisor. And then there are those call-outs which simply defy belief. Mervyn was recently summoned to buy someone a pack of cigarettes; others were called to fix washing machines or silence burglar alarms.

"There is no such thing as a busy period at this time of year," Emma tells me as we pull out of the station at the beginning of our shift. "The second you drop one patient off, another call comes in – from the beginning to the end of your shift, no matter the time of day or night. And you never know what you're going to find when you arrive."

Saturday, 17.15: I am met at Camden ambulance station by Gary Cross, the duty manager, in plenty of time for a 6pm kick-off. Actually, Gary explains, our start-time has been moved back to 7pm. Emma, the emergency technician and driver for the evening, came across an accident on the motorway on her way home to Bedfordshire this morning – like a number of staff here, Emma commutes two hours each way to work. Such is the duty of care of an ambulance worker that she was bound to stay at the scene until paramedics arrived. Having finished her shift just after 6am, she didn't get home until 11.30am – so in order to squeeze in some much-needed rest, she will be arriving an hour later than planned.

This gives me plenty of time to quiz the off-duty workers preparing for their shifts. Jon Stewart, a 31-year-old rapid-response driver, explains that his job entails working alone from a car. He will usually arrive at the scene before the ambulance. Whereas ambulance workers only tend to wear a protective "stab vest" when attending a scene at or near "a flagged address" – there are currently more than 1,400 addresses on the London data-base where tenants are known to be violent – rapid-response drivers, who work without back-up, wear them all the time. Violent assault is a hazard of the job.

In 2008-2009, there were 54,758 reported assaults against emergency medics in the UK. Everyone I speak to during the course of the evening has had such an experience: Jon recalls being attacked with a brick by an escaped psychiatric patient a couple of weeks ago; Emma was doing a house-call with a female colleague recently when the patient turned and punched her co-worker in the face, in front of police. Earlier this year, in another part of London, Suzanne O'Rourke was shot twice after treating a patient. She only survived because she was wearing a protective vest.

18.15: Emma arrives at the station. Having changed quickly into their uniforms, she and Mervyn explain some of the hi-tech gadgets along the dashboard of the ambulance. One of the newest vehicles available, it has complex touch screens, sat-nav devices and radio- control systems in-built. Emma explains the pioneering "Eastings and Fairings" system: the second a call is connected to the operator, its general area is picked up by radar and a street map appears on screen. That way, the ambulance crew can start moving towards the scene even before an address is given by the caller. It is all very clever. That is, until something breaks down. And then it's back to pen and paper and good old-fashioned map-reading.

There is just time for a quick equipment check in the back of the van before our shift officially begins. Often, Emma says, this is not the case and you have to head straight out on a call, hoping that everything is in place. According to a conversation I overhear in the staff room, there are rumours that a number of the older ambulances are poorly equipped, and that some staff members are even buying their own equipment to ensure they have the basic kit they need to do their job.

I also overhear passing comments about poor pay and conditions. Given that salaries for ambulance staff in London, where wages are generally higher than elsewhere in the country, start at just over £20,000 and peak at less than £38,000 – and that they are expected to serve an average of 37 hours a week, typically spread over four 12-hour shifts, with workers rarely having time to take their allotted 45-minute break and shifts regularly over-running – it is hardly any surprise.

Back in the ambulance, it is one minute before the 7pm start-time, and Emma climbs behind the wheel, with Mervyn by her side.

19.00: Ten seconds in, the first call comes through: "36-year-old female, unconscious". Flashing lights on, Mervyn calls out directions from the sat-nav. Soon we are flying through Camden Town, where pedestrians regularly run out into the road in front of us – we're moving at 40mph and have to break suddenly not to hit them. "People see an ambulance and they start crossing anyway," says Mervyn. "Maybe it's through panic, but I'm sorry to say I think a lot of people just don't care."

19.11: We arrive at the residential address. It turns out the patient is 26 years old, not 36 – details of incidents are frequently misconstrued during fraught 999 calls. She has recently come around from her unconscious state. She seems OK but the ambulance crew is obliged to take her to hospital for a check-up.

Filling out the first lot of paperwork in the back of the van, Mervyn says he's tired. He's been on a "bad rota" and has only been getting three or four hours sleep between shifts. Aged 32, Mervyn graduated from a three-year degree to become a paramedic just two months ago. He had been training on the ground along the way, but is still fairly new to the job. Emma is a medical technician – a less-qualified position than a paramedic, officially making her his second-in-command – and has been working on the ground for nine years. At 29, she says she still sees things she's never witnessed before – "but there's no substitute for experience".

In his short time with Camden ambulance service, Mervyn has already seen a lot. Last week, he was called to the scene of a stabbing. When he arrived, the 20-year-old victim was dead. What does it feel like to be faced with such a situation? "I don't know how to describe it," he sighs. "You have to develop a certain level of detachment. If you took everything home with you, you'd go mad. Although, that said, sometimes you get home and it's not as simple as simply switching off."

19.58: "We're picking up speed, as this is a bit of a dodgy one," shouts Emma from the driver's seat. The second call comes just seconds after Mervyn presses the green button informing control room that we are clear of the last patient. We're now doing 50mph along narrow residential streets, picking up pace on the flyover; cars continue to swing erratically in front of us, refusing to pull over. "A one-year-old male has had a knife dropped on his head," Mervyn explains. "At this stage we don't know whether it's an accident or not."

Such calls, clearly, have to be handled with caution. Emma has seen two cases of child abuse in her time. "A lot of the time, you can just feel something isn't right. It's the subtle things, you just have to act on your instincts." It's a big responsibility, particularly when ambulance crews aren't trained to detect such incidents. Still unsure what they'll meet inside, Emma and Mervyn tell me to wait in the van while they enter the house. When they emerge, minutes later, the child, his head pouring with blood, is screaming with pain. His mother, equally distraught, holds the boy in her arms all the way to the hospital.

She says she is in the middle of a messy divorce, and has just moved into a new home and her head's all over the place. While washing up, a sharp kitchen knife dropped from where it was stashed at the top of a cupboard on to her child's head. She doesn't know whether it landed blade-first. Soon we are racing towards St Mary's paediatric A&E Mervyn in the back attempts to console the little boy with a blown-up surgical glove on which he has drawn a felt-tip smiley face. He and Emma accept that this was an accident, and decide not to submit a special Social Services form.

22.14: "This one could be a stabbing, hold your fire before getting out," Emma calls into the back. We are pulling out of St Mary's A&E again, having just dropped off a three-year-old girl who'd been having a fit, when the next call comes in. Soon we're swinging through central London, towards Berkeley Square in Mayfair. There is no sign of the stab victim at the address given. A policeman explains that the man disappeared in the back of a car. Mervyn fills out yet another load of paperwork before we move off – even if the victim isn't present when you arrive at the scene, a report still needs to be made. Several more assault calls come over the radio during the time it takes to complete the forms.

22.49: Just as we answer yet another assault call and flick on the flashing lights, the job is cancelled. We're needed to attend a 76-year-old male who is coughing up blood on the 11th floor of a block of flats. Again, we're hurtling along the Westway, the city lights flashing past. I am starting to feel a little faint.

23.14: An overpowering stench hits us as we enter the flat's front door. As well as vomiting blood, the man in question is partially disabled and diabetic and hasn't taken his insulin for four days. He has been constipated for weeks and his blood sugar level is near-fatally high. He says his insulin is in the fridge, but there is nothing but mould to be found. The kitchen is crawling with maggots – the man's private carer hasn't been for three weeks. He says that he pays her to come for an hour a day, but when she does show up, she only stays a few minutes.

A vulnerable person's form needs to be filled out. "He can't come back to this flat," Emma explains. Until a permanent arrangement is made, the man will be kept in hospital as a social case. Before he is moved there, a physical examination needs to be made in the back of the ambulance, in the courtyard of the estate. It takes some time, and meanwhile we're blocking in a white van.

23.48: The van driver steps out and slams the door. He strides across to the ambulance, and without a word opens the passenger door, where I'm sitting; the paramedics are conducting a physical examination in the back. The man leans in, inches from my face. "I'm late. You need to move." I explain that we have an ill patient in the back and will be moving soon. He repeats his words and clenches both fists. As he walks back to the van, I lock the doors, suddenly aware of how vulnerable medics on the frontline really are.

Sunday, 00.09: There is no bed at Hammersmith A&E. We wait with the old man, who is wincing with pain on his stretcher, for just under an hour before a space comes up. As he is wheeled into a cubicle, a drunk woman screams abuse at the nurse on the next bed.

I meet a paramedic in the car park. "In this job, you often find yourself wiping your feet on the way out rather than the way in," he says.

In the ambulance, Emma explains that this was one of the milder cases she has referred to the Adult Protective Services in her time. Neglect is rife in the elderly care system, it seems. "You get some terrible carers," she says. "It's a hard job and I don't want to put them down, but some of the things they do – or don't do – beggars belief."

01.07: We are on the way back to base to submit a vulnerable person's form when we spot the woman who has come off the child's scooter, in the middle of the road. She is not only concussed by the time she comes round, but also drunk, and won't co-operate with the paramedics. She struggles and screams as they try to fit her with a spine brace to prevent potential damage to her back. It is icy cold and the rain continues to beat down; 45 minutes passes before they to get her into the ambulance.

02.02: The Royal Free Hospital keeps a stock of vulnerable person's forms, so, having dropped the scooter woman in A&E, we no longer need to go back to base to pick one up, and head out once more.

02.22: "Shooting, Wembley, 25-year-old male." We are in the lift on the way to a suspected overdose in a flat in Camden when the call comes across Emma's remote radio. "Another quiet night," she says.

Two policemen meet us in the hallway to the flat. Inside, a woman in her late twenties is curled on a bed. She has been suffering from depression. She hasn't overdosed, it turns out, but hasn't eaten anything for three days, and her blood-sugar level is so low that she is in danger of lapsing into a comatose state. She refuses oral glucose to lift her levels, and won't go to hospital.

Emma makes her sugary tea and toast. Mervyn sits talking with the woman at the kitchen table, trying to stop her drifting off. She picks at the bread, but 45 minutes later her sugar levels haven't risen, and eventually she is persuaded to spend the night in hospital, with intravenous glucose.

"A&E isn't the place for her," Emma says as we leave the hospital. "But where else does she go? We don't have the time to sit and talk to her for hours, and there's no GP surgery available between 10pm and 6am. It's a massive loop-hole." Paramedics get a lot of mental-health patients, Mervyn adds, but aren't trained in that area: "There's not a lot we can do except talk to them."

03.50: "Oh no," Emma sighs as we pull out of A&E. A young woman has been raped in W1. Seconds later, the voice from control adds: "Ambulance now on scene."

03.51: "Eighteen-year-old, female. Unconscious. Park Lane." We find her 10 minutes later, curled on the floor inside the club. Her gold sequin mini-dress is hitched around her waist; vomit hangs from her hair and is smeared across her face. She has no bag, no friends, no coat. She was found in the loo at 3.30am and hasn't uttered a word. The caller guessed at her age, but no one knows her name.

04.35: University College Hospital's emergency waiting room is a sea of slumped bodies. Bloody footprints stain the lino floor. A 23-year-old stab victim is being wheeled into a cubicle, next to a man who is dripping with blood. He's been bottled several times in the face.

We never find out what happened to the girl.

05.40: By the time a bed has been allocated and another batch of paperwork is complete, I am struggling to keep my eyes open, my head is swimming. I am tired, frozen, hungry, thirsty and emotionally drained. Mervyn yawns in the passenger seat. We set off once more.

05.42: The final call sounds straightforward: a young man has dislocated his shoulder. Inside a sheltered housing unit, we find the 22-year-old man is recently paralysed from the waist down, the result of an infection in his thigh. Ten minutes of gas and air doesn't touch the pain in his shoulder, and a suitable vein in which to inject morphine cannot be found. The man apologises, explaining that it's because of his paralysed state. He says he can't go to hospital, he has a phobia. Finally he concedes that he hasn't got a choice, if he wants his shoulder put back in place. It takes more than an hour to get him from his bed to a stretcher.

I stand outside for 15 minutes and collect myself while they tend to the patient. I stick my fingers in my ears to block out his cries of pain.

06.28: We arrive back at base. There wasn't time to take our allotted 45-minute break, so we finish 30 minutes early. Though by the time all the paperwork is filed and ambulance handover complete, it is nearly 7am anyway. Several of the workers I met earlier on are having a cup of tea before heading off. The night's events are going round and round in my mind. "OK?" they ask. I nod, but I'm not feeling OK. I say goodbye and make my way towards the door as Emma heads towards the showers to refresh before starting the long journey home.

Outside, I lean back against a wall, and stay there for several minutes, eyes closed. Soon I hear Mervyn shouting goodnight to his crew-mate from inside.

"Night Merv," Emma replies. "Same time tomorrow!"