I’m sure I am not the only one who has harboured a dream of enjoying lunch at St John with Fergus Henderson. On a long list of ideal dinner party guests, Fergus has always been right at the top, name in capitals, written in permanent marker. I'm due to have lunch with him on Monday but the Saturday before, I'm struck down by a stomach infection that has me doubled over with waves of agonising cramps. By 3am on Sunday morning, I've finally found my way to a doctor. 'Gastritis', he says. 'Take these H2 blockers and some Rennies and don't eat anything rich. Drink lots of water.' He ushers me out. 'Oh, and no alcohol. Obviously.'

You never quite know what to expect from your heroes, but as we set up on Monday morning to take a few photographs, Fergus arrives resplendent in the uniform we’ve come to know so well – walking stick, well-worn tan derbies and sartorial pinstripes from head to toe. After much hand-shaking and some pleasantries, we suggest that it would be nice to photograph him with a drink in hand. ‘A very good idea!’ he says. He fetches three glasses of St John Pinot Noir from the bar – one for each of us – and proceeds to hand them out. I briefly consider my mortality. Drinking on a workday morning makes us feel a little like giddy school children, but Fergus – nursing a hangover from St John’s anniversary celebration the night before – insists that it is purely medicinal. ‘Things are a little blurry at the moment,’ he chuckles. ‘Pinot Noir should put that right.’ Bolstered by a second opinion from Doctor Fergus, I decide it would be rude not to join him.

Yesterday’s festivities were in celebration of St John’s twenty-fifth birthday – a monumental achievement for a restaurant that has, in retrospect, done more than any other to lead British food out of the doldrums and into something resembling a golden age. Or an iron age, perhaps; not in the sense of backwards-ness but in functionality and rigour. St John has never had delusions of grandeur – Fergus and Trevor have gone about their business quietly, believing simply in an ethos of sustainable eating and the importance of food and drink to the spirit. It seems that over time, we have all come to treasure that dining philosophy; the turnover of restaurants in the capital is as fierce as it has ever been, yet St John remains, a monolith of wood and whitewash.

Fergus stopped cooking at St John a long time ago – the result of being diagnosed with Parkinson’s disease in 1998 – but he’s still a consistent presence at the restaurant. If he’s not having lunch, you may well find him entrenched at one end of the bar with an Eccles cake and a glass of Madeira. Sometimes he’ll tour the kitchens and chat to the chefs. ‘This is still my second home,’ he says as he climbs the wrought iron steps to dining room – a procession to lunch that he must have made a thousand times over.

Trevor Gulliver – his long-time friend and business partner – joins us too, and appears to be coping rather better with his hangover as he sits down next to me and proceeds to pour me a glass of wine. Trevor and Fergus met in 1992; Fergus was working at The French House in Soho with wife Margot and Trevor had just sold The Fire Station – a bustling bar in Waterloo. ‘We had a mutual olive oil supplier,’ Trevor explains. ‘He told me I should meet the young guy working above the French. And that was it! We had lunch – he came to my house once, I think. We used to eat at the French – lunch would sometimes go on quite late!’

I feel rather torn between my dual roles as both interviewer and lunch companion, but I’m keen to ask the pair how they view St John in the light of the last twenty-five years and if they’re proud of the legacy the restaurant has created.

‘It’s been an extraordinary journey,’ says Fergus, ‘but legacy is a dangerous word. Legacy is something someone leaves behind.’

Fergus remains tremendously thoughtful, but his Parkinson’s and subsequent deep brain stimulation treatment has left him a rather quiet speaker. Often he will start a thought and Trevor will pick it up and run with it – the two are as good as symbiotes after nearly three decades together: ‘It's not about legacy at all, it's about now,’ Trevor explains. ‘As Fergus says, nature writes the menu. As long as the earth continues spinning, we will continue doing what we're doing.

'We didn’t want this book to be an epitaph or a pat on the back. It’s a cookbook first and foremost, with some insight into how St John works.’

The message is loud and clear – St John isn’t going anywhere. Indeed, the restaurant will soon open its first outpost in Los Angeles, where Fergus fandom is rabid thanks to the influence of the late, great Anthony Bourdain, who was never shy in declaring St John as his favourite restaurant in the world. Fergus is still immensely proud of the St John family tree; James Lowe at Lyle’s, Lee Tiernan at Black Axe Mangal, Justin Gellatly at Bread Ahead, Tim Siadatan at Trullo and Padella and Anna Hansen, recently of the Modern Pantry have all gone on to forge their own paths, but the hallmarks of their St John education are unmistakable. ‘Now our family tree spreads far and wide,’ he writes in the introduction to The Book of St John, ‘often in quite unexpected directions, and I am humbled to bask in the dappled light beneath its leaves. I have often said that I feel like a mother hen with a long and busy line of chicks going forth to establish their own nests, building new things from our branches.’

The marking of St John’s twenty-five year anniversary is not a passing of the torch. Instead, it is recognition of the fact that St John has been doing what it does for twenty-five years, regardless of trend and opinion. St John is part of the furniture in Smithfields now, but twenty-five years ago this area was urban wilderness. ‘It was a wasteland,’ Trevor remarks. ‘You wouldn’t recognise it.’ It was Trevor that found the site. By today’s standards it was far from ideal – the entrance was easily missed from the street, and the space was long and narrow, geometrically impractical for a restaurant. The pair loved it regardless, whitewashing everything, bringing in wine glasses and crockery and opening the doors in 1994.

Much of the same crockery, glasses and furniture is still there now. ‘We had an unfortunate incident when Jonathan [Woolway, St John’s executive chef] decided to varnish all the tables,’ says Fergus. ‘We didn’t much like it, so he had to go round and sand them all down again.’ There’s an old dresser next to the kitchen that houses spare plates; one of the doors is hanging off at a jaunty angle. You wouldn’t look twice at it if you were at your local tip, but it’s an important part of the fabric of St John’s dining room. The stubby, workmanlike Riedel wine glasses have long since gone out of production, but Trevor continues to bombard the Austrian glassware giant in the hopes of getting a new batch made up. All these things might seem a bit frivolous, but changing the glassware is no different to changing the recipe for the restaurant’s legendary bone marrow, parsley salad and toast – these things are all vital to St John’s permanence.

Trevor has an inexhaustible bank of timely anecdotes and he rattles into another one before I can squeeze out a question about veganism. No matter – I already know what the answer will be. He tells a story about a paparazzi who arrived at St John one morning in the 90s: ‘It was when we started serving carrot, egg and aioli,’ he explains. ‘It got a bit of attention in the press, and a guy from The Sun turned up looking for a scoop. He took a look at the menu and frowned a bit, and then asked if we could put the price up! They wanted to write this sensational story about a London restaurant charging crazy money for a carrot, but it was only £2.85!’ The idea of serving a nice carrot with a nice egg and a bit of aioli in the ‘90s was shocking enough that The Sun thought it would sell newspapers. Today, there are restaurants and gastropubs all over the country that happily sell something similar, run by chefs who have been inspired in one way or another by the gospel of St John.

‘When we first started, there was no zeitgeist about what we did,’ Trevor explains as he tops up my glass. ‘Twenty-five years later, it seems that we are the zeitgeist.’ The difference is subtle, but significant. St John hasn’t changed; the culinary world has.