If Heston Blumenthal had intended the design of his new restaurant to be a physical metaphor for the risks he is taking, he couldn't have done a better job. He knows full well that Dinner by Heston Blumenthal, at the Mandarin Oriental Hotel in London's Knightsbridge, is a big gamble: it's his first serious restaurant since he quietly opened the now three-Michelin starred Fat Duck at Bray in the mid-90s; it has a complex culinary agenda built around the modernisation of historic British food; there is the commitment of millions of pounds from his backers. His whole brand is going to be hugely exposed, and literally so. The kitchen is a vast glass-walled box right in the heart of the dining room. At Dinner there will be nowhere to hide.

So it matters who the £75-a-head diners will see inside that glass box. "I've made sure to tell people I'm not the one who's going to be here," Blumenthal says, during a visit to the fast-advancing site. "I'll be around, but basically I'm in Bray." Instead of his shining pate and fast-forward-to-the future glasses, they'll get a soft-cheeked Dorset-born thirtysomething chap called Ashley Palmer-Watts. "I'm utterly confident of putting Ash out under my name," Blumenthal says. "It was always going to be Ash. I simply wouldn't have done this if it wasn't for him." Palmer-Watts returns the compliment. "The one person I would not want to let down is Heston," he says.

To describe theirs simply as the relationship of mentor and protege is, however, to misunderstand the culture of the Fat Duck. They both accept the titles – "I sound like a proud dad, when I talk about him like this," Blumenthal says at one point. "It's great to have Heston at my shoulder," Palmer-Watts replies – but it goes much further than that. "You will thrive around Heston if you want to absorb, challenge and discuss," Palmer-Watts says. And if that makes Blumenthal's team sound like a priesthood, it may not be accidental. "We've evolved our own path to designing things," his most trusted lieutenant says, dryly.

It was not always this way. When Palmer-Watts joined the Fat Duck 10 years ago, the set-up was chaotic. There were no more than five cooks – today 45 work the various kitchens – and Blumenthal was still in his "complicated phase". He could get supremely angry during service and start smashing plates; his small brigade once put on crash helmets when they saw the storm coming. Not that Palmer-Watts knew any of this. "I went to the Fat Duck with a friend when I was 21," he says. "It was just so different. He was doing this chicken dish which cost £75 to make. He was selling it for £60. The whole thing was being marinated in truffle jus, Madeira and port. I just thought, 'I have to work here.'"

Palmer-Watts grew up in Dorset, and caught the cooking bug in his teens courtesy of a local restaurant that became something of a refuge when his parents split up. With the owners, Geoff and Linda Chapman (who went on to run Lumiere in Cheltenham), he travelled the world and ate well. From there he took stages – short unpaid periods of kitchen work experience – at everywhere from Le Manoir aux Quat' Saisons to Gordon Ramsay's Aubergine, which even offered him a job. But it was the Fat Duck he wanted, so he went to work on a friend's watercress farm, while he waited for a vacancy. "That impressed me," Blumenthal says. "I didn't have a job to give him at the time so he just waited. I told him to call me every day." Why? Was he testing him? "No, I was just too busy. I needed him to keep reminding me."

So what kind of employee was he? "Very quiet, which was odd. The mixture of my then less-than-calm demeanour and the hours I had to work meant that I was used to people around me who were slightly strange because they were the only ones who would work for me. And then here was this polite lad who had caught the food bug." It came down to some very simple virtues. "He was never late; he wanted to talk about food. Ash was interested."

In the early days the Fat Duck culture revolved around hard kitchen services and equally hard stints in the gym each afternoon. They never got enough sleep. "I remember telling him in the gym one day that he had a great future if he stayed with me." He meant it. By the time Palmer-Watts was 25 he had been appointed Blumenthal's head chef. "He's extremely encouraging," Palmer-Watts says. "He will push me. He's worked out that by making me uncomfortable, I will produce my best work. It's about applying pressure."

As the business expanded, so did the job of keeping track of what was going on in Blumenthal's domed head. "I'm a bloody nightmare to work for. Right now with all the TV work and the restaurants and other contracts we've got 600 dishes in development. And I'm anal about everything. Take a lemon tart. It will be about the thickness of the pastry, the setting temperature of the filling within half a degree. There's an awful lot of pressure." Palmer-Watts clearly thrived on it. "A dish doesn't go on the menu unless it's right," he says. "It's got to have more to offer than just be a plate of food. It's got to be a multi-sensory thing." Or, as Blumenthal says, "Over the years Ash has developed my traits, that obsessive compulsive attention to detail."

That, they both say, lies at the heart of the new venture. Blumenthal has said many times that if he was going to do a second major restaurant, it had to be distinct. Two years ago, Palmer-Watts stepped down as head chef – or, as Blumenthal tells it, was dragged out of the kitchen – to work on other projects including the makeover for Little Chef. "That was all about jeopardy. It showed him there is another world out there." Palmer-Watts is less enthusiastic: "It was absolutely horrendous." By comparison the new restaurant – despite its 100 staff, and 100-plus seats – looks like the proverbial walk in the park.

Blumenthal has become intrigued by Britain's culinary history and, with food historians from Hampton Court and elsewhere, has been developing dishes that have a British rather than French root. This is what defines the new menu. Palmer-Watts starts a slide show on his iPad: descriptions of dishes from the 18th century, in flowing serif fonts, followed by their makeovers: a turkey sausage recipe that has become a turkey boudin enclosing nuggets of bone marrow, alongside braised cockscombs and girolles; a beef royale, that will demand the braising of short rib for 72 hours, to be served with a smoked anchovy and onion fluid jelly; a ragu of pig's ear from 1750, with onions cooked in smoked duck fat.

So whose dishes are these? "Ours," Palmer-Watts says. Blumenthal concurs. "The whole process has been very much us. Ash is a very good foil for me. I'm like the kid at Christmas who wants the upside down mountain with the flashing lights and the plane flying round the top. And Ash is the one who says no, Billy, they don't actually make those." Does Blumenthal ever fear losing him? "I don't even think about it. Why the hell would he go anywhere else?"

Which makes Blumenthal Enterprises sound a little like John Grisham's The Firm: people only join, but nobody leaves. Then again, being shown round the as yet undressed restaurant, with its solid but modern panels in leather and wood to play up the English theme, its views over Hyde Park, the shiny kitchen, complete with a rotisserie with a polished cog mechanism, custom-built by a Swiss watch company, it is hard to imagine why anybody would say no. "I have tried to put the fear of God into Ash," Blumenthal says. "I want to make him shit scared, but also very excited." It seems to have worked. "You think about doing other things," Palmer-Watts says. "But I like playing at this level. I like the way we think and what we do."

Though, of course, it isn't "we"; it really will be Palmer-Watts. He slips off to the kitchen, which has only been handed over by the contractors while we are there. He turns on the induction hobs and, for the first time, plates up a dish – tranches of marinated mackerel, a smear of anchovy sauce, pickled vegetables – so that we can photograph it. Afterwards Blumenthal and Palmer-Watts, mentor and protege, head chef and collaborator, friends, stand over the plate and pull it apart with their hands. They nod. They do not say much to each other about it. They really don't need to. They are a long way beyond that. OFM

Dinner by Heston Blumenthal opens at the Mandarin Oriental Hotel, Knightsbridge on 31 January