"Next year we'll have been here 20 years," says Fergus Henderson, taking a seat downstairs at St John in Smithfield and gazing around in mild disbelief. "Each day is different, so it never grows dull. But I like the way there's a sense of permanence – it's become part of the fabric of the city, like a chemist or a bank."

St John has become intrinsic to British dining. Few restaurants have done as much to make us appreciate our national cuisine, and Henderson, with his respectful and pared-down approach to ingredients, continues to influence chefs at home and abroad.

He shudders at the notion that he's responsible for any trend, and tells me that his nose-to-tail philosophy – making use of all parts of the animal – is sometimes interpreted in "weird" ways. "Someone was doing a tasting menu in New York and had half a raw sheep's head. It was all wrong – you like offal so you've got to be hard. No. What we do here is I think very feminine, we have a light touch. You should be nice to your offal."

He's also quick to emphasise that developments in the restaurant world have little impact on the way we eat at home. "Everyone's watching cooking shows on telly, but butchers are disappearing. Supermarkets are selling us pink-in-plastic [Henderson's term for heavily processed meat] and unnaturally ripe fruit. Chefs are a bit more glamorous: that's changed. But the change isn't as big as it should be."

Still, Henderson is clearly proud of what St John has achieved. About the many chefs he's had under his wing over the years, from Anna Hansen to up-and-coming names such as the Young Turks' James Lowe, he says: "I feel rather hen-ish. It's nice when people go off and lay their own eggs."

Right now, though, he's excited about lunch. The first grouse of the year has just arrived and he's heading off to inspect it.