Holly Williams joins the staff at the online grocer Ocado ahead of the last-minute Christmas-food rush at the firm's mammoth warehouse

Getting the right food is rarely more important than around Christmas – especially if those hard-to-please relatives are coming by. And increasingly these days, that's as likely to mean a visit to the PC as the supermarket.

Buying online means there's no going feral in the aisles, snatching tins of goose fat from other frazzled designated chefs. No being judged for the terrifying volume of gin you're buying – and no having to lug it up flights of stairs. It also means no struggling with spinning tyres in a blizzard, and no having to pull that family-sized bird and several kilos of potatoes home on a sleigh.

So the biggest relief in many homes this year won't be when Santa and his sack pop down the chimney, but when a colourful van pulls up outside on its nonchalant winter tyres, stuffed fuller than a turkey with bags of groceries…

Since launching in 2002, the online-only supermarket Ocado has increased its range from 8,000 to 36,000 products. At Christmas, the average customer's single-delivery spend with the e-tailer doubles from £110 to more than £200. Its partner being Waitrose, it naturally offers fancy-pants exclusive ranges "to make Christmas special" or for that "unique centrepiece". (A £160 six-bird roast, anyone? Apparently so: it's already sold out and the company is having to order in more.) But Ocado is also insistent that with its value own-brand, it's now rivalling Tesco and Sainsbury's on basics.

The warehouse in Hatfield measures a million square feet (Benjamin McMahon)

Such a big operation needs its hordes of elves – not to mention elaborate computer systems. On visiting Ocado's main warehouse in Hatfield, outside London, to see how they do it, I'm faced with a vast place, filled with swishing cranes pulling products off endless stretches of shelves…

OK, not quite endless – even if looking down through metallic grid floors can induce a Matrix-like vertigo – but the Customer Fulfilment Centre (CFC), to give its pompous full title, encompasses a million-square-feet – that's 13 football k pitches. The company has recently opened another CFC in Dordon in the Midlands, which it will share with Morrisons, whose online delivery system it will provide from early next year.

All around us, colourful plastic boxes – or totes – trundle by on hissing conveyor belts, sliding up and down slopes, taking corners, even scooting down a helter-skelter corkscrew. To evoke childhood Christmases past, it's like a mammoth Scalextric track looping round a Meccano set. I express an interest in getting inside a tote and going for a little ride around, but alas, no luck.

To be fair, I'd probably muck up the carefully controlled system. "Because we dispatch for a one-hour delivery slot, everything needs to be perfectly in tune," Allan Carruthers, the general manager who's been with Ocado for 10 years, tells me. "The frozen needs to arrive at the same time as the ambient or chilled. If you order lots of small items, a tote can take three hours to go around the warehouse. So the system has to be clever enough to ensure they arrive at the same time."

He points to a busy-looking man in a control centre, watching a near real-time computer simulation (complete with console-style controller) that tracks the totes' journeys. He has the wonderfully Zen-sounding job title of Flow Manager, "like a flight controller".

The conveyor belts act like a giant Scalextric track (Benjamin McMahon)

The company has an IT team of around 700, and complicated codes and algorithms help everything run smoothly, from a Tetris-like management of storage space to the optimisation of delivery routes. Every item is electronically scanned, repeatedly, by the 1,500 factory-floor staff, who wear portable scanners on their sleeves; all very sci-fi. "It fires a laser so you can scan barcodes; it has to be handheld because of my job, walking up and down replenishing the aisle. I'm basically a shelf-stacker," explains Andy Kadir-Buxton, on staff for nine years. "I used to be trolley-picking in the freezer at one stage, I had my Chairman Mao suit on…"

Ah, the freezer. At -26C, workers have to bundle up in padded suits. I pop in to look at the mountains of frozen turkeys – though I'm rather more interested in the crate full of ice-cream. But even that can't tempt me to stay long. Those working in the freezer have one of the hardest jobs; it's too cold for the cleverly automated conveyor belts, so they have to hand-haul items in trolleys. And it's so cold that their balaclavas quickly become covered with a Jack Frost-like beard of ice.

The system that controls the whereabouts of the totes along 25 miles of conveyor belts has to be clever enough that pickers are able to box up frozen goods and fresh food at the same time (Benjamin McMahon)

Back in the ambient zone – tropical, by comparison – we see more "personal shoppers" (or "pickers" as staff refer to themselves) at work, alongside another clever piece of kit, the Order Storage Retrieval (OSR) system. Rather than the pickers stomping up and down the aisles, the OSR mechanically hands them a product from one of 40,000 shelf locations at the same time as a customer tote goes by. The picker simply grabs the Christmas pudding or giant bottle of gin and pops it in the customer's bag. It's unique in the grocery business.

"The technology is used in pharmaceutical warehouses – no other supermarket uses them like this," says Carruthers. If it is a somewhat unsettling marriage of man and machine, it is speedy, allowing a picker to pack 800 items an hour. That's a lot of shopping.

It also means workers can be tracked on their speed – a somewhat queasy feature, given the recent Panorama investigation that accused another online giant, Amazon, of mistreating its staff by putting them under relentless pressure to collect items within a certain time, or face disciplinary action. Amazon responded that "the safety of our associates is our number-one priority" – a statement Ocado echoes.

"The welfare of our people is extremely important to us and we're very proud of the working conditions we offer," it says. "We designed each CFC with our personal shoppers in mind, and each one has around 25 miles of conveyors that move k product around the warehouse, minimising the distance our employees need to walk during their shifts. Our picking stations have been ergonomically designed to ensure that personal shoppers are able to pick groceries in comfort."

The pickers wear portable scanners that allow them to scan barcodes (Benjamin McMahon)

Of course, as a shopper, you don't get to see any of this – the sole interaction is with the van drivers. Sorry, the customer service team members. And getting on with customers clearly is an important part of their job; they have to prove their people skills to pass the probation period. "I've done training for seven years; you tend to know if someone's going to fit in," insists Leigh Sharman, one of Ocado's 600 drivers. "Some people can just naturally talk to people."

Tact – and soothing customers who have been waiting in a little too long for their delivery – may be especially necessary round the busy holiday period, although the two drivers I meet insist customers are mostly just relieved to see them. Older customers often try to tip them £1 for a cup of tea, but tips aren't expected, even at Christmas. I suggest being slipped a mince pie might not go amiss, and driver Nicholas Hill recalls once having a pack of beers pressed on him by a customer particularly full of goodwill. "Which was nice, but obviously I'm driving…"

They take the business all the more seriously in the run-up to Christmas, working right up to lunchtime on 24 December. On, for instance, go the winter tyres: "We do very remote places – country roads where you can just about get one vehicle down there. I did do a few in the snow last year, but it was all right," says Hill sanguinely, while Sharman explains they keep buckets of grit and a shovel in the back, just in case.

The order storage retrieval system, which allows staff to pack up to 800 items an hour from 40,000 shelf locations (Benjamin McMahon)

"When there's snow, [customers] think you might not get there and they're pleased as punch to see you – especially if you have the Christmas bits," he adds. And staff have had it instilled in them how important it is to have all the "bits"; not delivering the parsnips on Christmas k Eve is unacceptable. In December, the drivers do a manual double-check of the orders inside the van, rootling through bags to make sure each sprout is present and correct.

But who decided what goes in those bags? The customer? Well, maybe, but even if online shopping helps you resist till-side temptation, the website is designed to divert the eye to products you might not have thought you needed.

For Ocado, that means unique items. Senior trading manager Kevin Hancock, who's been with the company from the start, says that the item for 2013 is the Christmas dinner cake, made with carrots, courgettes, sprouts and parsnips, with cranberry icing. "It's about talking to suppliers, getting them to give us stuff they won't sell anywhere else – the weird and wacky," says Hancock. "That's where our jobs become really quite fun. As a buyer, you should know within two minutes when you see a product whether it will sell. Your gut tells you instantly."

Drivers Leigh Sherman (left) and Nicholas Hill double-check every item (Benjamin McMahon)

Another hit has been a Malteser Christmas pudding, for those who like their desserts particularly chocolatey. And Ocado's been trying to corner the market on not-another-nut-roast options for vegetarians too, stocking a chestnut, fennel and cranberry loaf and a mushroom Wellington. Hancock is also rather excited about its Brie de Meaux: "We see a monumental peak in cheese [at Christmas]," he says, gloriously. And it's not just cheese: by late November they'd already shifted 12,000 bags of sprouts, 150,000 pigs in blankets, and 7,500 Christmas puds.

Booze also gets a bit special; wine buyer Freyja Kenny explains that, in the Christmas battle of the supermarkets, "Booze is where everyone trashes the price to get people through the door. People win through booze, basically."

Kenny's tactic is to focus on exclusive labels: "People don't want Sainsbury's or Tesco on their bottles." Her gimmick this year is the boozy Advent calendar, with a dram of 24 different whiskeys or gins: genius or irresponsible, depending on your view on daily tippling. And, true to its toff roots, Ocado still shifts a lot more champagne than most supermarkets. This year, it's selling not only magnums of champagne, but double-the-size Jeroboams.

Probably not one to put in your "favourites" section on the website for the weekly shop, but this something-special approach seems to be, well, delivering. Or maybe it's just that the customer likes to feel that massive relief of not having to go to a supermarket on Christmas Eve… I'll raise a glass of fizz to that.