Just add water

Hot tubs are big sellers. To find out why, Julia Stuart puts one in her back garden - and is amazed by just how many good friends she has

January. Rubbish, isn't it? After years of suffering from a post-Christmas misery that would send me sobbing into the carpet, only to be further tortured with a face full of pine needles, I've finally found a way of tripping through it in a state of utter bliss. No longer will I be making those urgent calls to the Australian embassy begging for an immigration form. My new-found happiness may involve being buzzed by police helicopters and having to see my friends almost naked (good God, they're hairy), but it's worth it.

I fall in love with my six-seater hot tub the first time I see it suspended between the washing-line and a telephone cable as it was being craned over the back fence. There is something marvellously satisfying about the stonking great size of it, the way the sun glints off the Cellophane wrapping around its redwood panelling and - best of all - the sudden movement that it produces at my neighbours' windows.

No one that I know has one. No one that I know knows someone who has one. Even Dave, the delivery man from HotSpring Spas, hasn't got one. My toes curl in anticipation.

The hot-tub craze first started in the 1960s in America, primarily in California where oak barrels and vats from nearby wineries were used. They are believed to have been inspired by the Japanese custom of bathing in freestanding wooden tubs filled with hot water, witnessed by servicemen during the Second World War. Redwood tubs then started to appear, which often leaked and were eventually replaced by today's acrylic shells. The early models simply produced bubbles, whereas today's varieties are built with hydrotherapy jets.

Now, it seems, everyone wants one. According to the British and Irish Spa and Hot Tub Association, sales are expected to have doubled in 2004. The reason is thought to be affordability, with models starting at around £2,500. And, presumably, word has got out about just how much fun can be had in one. Lady Julia Pilkington found the experience so explosively joyful that she was sentenced to a community service order after her neighbours in Southsea, Hampshire, complained of hearing orgasmic sound-effects as she cavorted with her lover. Now, more than 100,000 households in the UK have one and a growing number of developers are installing them in luxury properties, especially on roof gardens and the decks of penthouses.

Back in the badlands of south-east London, however, Dave shows me how to lock the mock-leather lid and the control panel because of my fear of local ruffians hopping over the fence for an uninvited dip. I can't even be certain that they won't make off with my £9,795 Vanguard, despite it weighing several tonnes once filled with 400 gallons of water, and being big enough to cover almost my entire lawn. In April last year, Tony Hill was understandably narked when thieves used a crane to steal his hot tub out of his back garden at his home in Bournemouth, knocking over trees and leaving snapped cables strewn on the lawn. "I'm furious. It's the audacity of these people," he said. "Plenty of the neighbours saw them, but they thought that they were workmen."

A hole is drilled in the doorpost of my kitchen for the cable, which will remain plugged in to keep the water warm. Dave then hands me a box containing several plastic bottles of chemicals and the all-important yellow rubber duck. It is essential to check the levels of chlorine and pH with a dipstick on a daily basis, and adjust them if they are out, he explains, solemnly.

It all depends, apparently, on how clean my friends are. I make a mental note to hose them down before I let them get in. While the level of chlorine can be increased just by adding a few teaspoons' worth, the pH increaser and reducer involves a rather scary mathematical calculation: four teaspoons of either changes the level by 0.2. Now is probably not the best time to admit to Dave that I wasn't allowed to take maths O-level. I suddenly develop a stress headache, but it is relieved by Dave's parting shot: "Watch out if you have a drink in there! The tub seems to heighten the alcoholic effect as it gets into the blood stream quicker."

He has set the temperature of the water, which is changed only every four months, to 39C. Tragically, it will take 24 hours for the water to heat up. I spend the evening, nose pressed against the cold bedroom window pane, waiting.

The following night, dressed in a bikini and a fishing-hat with ear flaps drawn down against the drizzle, I venture out to the garden and into the kind of weather in which people in films get murdered when their car breaks down. The ready button is glowing. I'm so excited I could cackle with joy as I unlock the cover and pull it back. However, when I put in a dipstick, I discover that the chlorine level is way off the scale. Dave warned me that he had "shocked" the tub with chlorine to kill anything nasty that might have grown during its transit from the States. As instructed, I turn on the jets, which are not that much louder than a rumbling fridge, and let it run to burn some chlorine off. Two hours later, the level is still too high. I lock it up again, go back inside and sulk in the bath.

Bikini back on, the following night, I venture out into the night again, pushing aside the wet branches. My boyfriend is wearing nothing more than my dressing-gown and a hat. I am worried that he will upset my now perfect chemical levels. I set the jets going and then turn on the underwater light to the disco setting and step in. I choose a corner seat and rest my back against a "moto-massage jet", which give a shiatsu-style rubdown.

The last time I was in a hot tub was in the Colorado resort of Breckenridge, where I caught snowflakes on my tongue after a day's skiing. I'd thought at the time that nothing would beat it. But I was wrong. There is something even more delicious about sitting in a hot tub chez soi after a bad day at the office, an orange glow coming from a nearby streetlight, mist rising towards the stars, with nothing to worry about other than the passing planes mistaking your disco setting for the landing lights at City airport.

The following day, wanting to experience my hot tub with a full complement of six, I make a few phone calls. Within minutes, every slot is taken for my Sunday-afternoon session. One friend from west London, whom I've known for three years but who has never made it to my home, happily travels for almost two hours to get here. I strongly suspect that had I said I was having a dinner party, half of them would have said that they couldn't make it - even if I'd promised not to do the cooking myself.

Armed with five bottles of champagne, and in a state of virtual undress, we make a dash for it across the decking and down the garden. My guests instantly find the handy glass-holders by the knobs around the side. Within 10 minutes, one of them vows that he will buy one. We spend a rather glorious afternoon, during which the conversation naturally turns to the more intimate aspects of life, despite the fact that some of them have never even met each other before. When making an earnest point to a chum sitting beside me, instead of lightly touching his forearm, I find myself gripping his leg.

Having baked Nigella's chocolate- fudge cake in honour of my friends' visit, the suggestion is made that we have it in the tub. One volunteer pads off to the kitchen and returns, dripping, with six slices. We eat it in the water and life suddenly reaches an even higher level of perfection.

Three and a half hours later, when my guests finally start to leave, it occurs to me that we have drunk rather a lot, and yet some of them haven't visited the lavatory even once. And that worries me.

The following day, when I test the water, there is no longer any trace of chlorine. I am also now rather anxious about my electricity bill and wonder whether I should put a collection box on the tub's wooden steps. But a quick call to Glen, manager of the showroom in Reading, reveals that the company guarantees in writing that their tubs won't cost more than £3.50 a week to run for an hour a day, every day, at 40C. He claims that other spas on the market have to have their main pumps turned on twice a day, which not only makes a lot of noise but pushes up the electricity bill. His tubs, however, filter the water through a cartridge.

Over the ensuing weeks, my friends continue to venture south of the Thames. During one telephone invitation, I am cut off with the words, "Say no more!", and my caller is on the Northern Line clutching his Speedos faster than you could say, "Would you mind showering before you arrive?". Two colleagues pitch up having forgotten to bring their swimming togs, and, unwilling to miss the opportunity, they venture into the water wearing select items of their gym kits.

I have now given up all efforts with the chemicals, despite the fact that it should take only 10 minutes a week. I am simply too lazy. And scared of sums. How much danger can my guests really be in? Quite a lot, apparently. Last September, a survey found that the deadly Legionella bacterium was present in 26 per cent of health-club spas. The Health Protection Agency deems spas to be responsible for 27 cases of infection in 2003, and three deaths. Oops. I'd better get my act together. I wouldn't want to kill off my friends - they've only just started coming round to visit.

HotSpring Spas (0800 085 8880)