Tucked away between Sloane Street and Harrods is The Capital Hotel, the discreetly elegant home of one of the finest restaurant kitchens in the UK. Both the hotel and its dining room are classic, old-fashioned Knightsbridge institutions where everything on offer is 5-star but nothing is too flashy. We sat in the window of the small but elegant, Nina Campbell-designed dining room, where we could watch the August tourists hurry with their shopping bags while we enjoyed some über-attentive service – impeccable and yet invisible.

The menu is grown-up, classic cooking, where contemporary twists of the gastronomic greats mean that any meal here is a serious treat. There is a very reasonable set-lunch menu (£24.50 for 2 courses), but you’d be better advised to order à la carte or perhaps try the ‘40th Anniversary Dégustation Menu’ (at a very reasonable £55 for four courses) if you really want to spoil your taste buds rotten.

The menu is small but perfectly formed, which meant there was little debate over what to choose. The ravioli of duck foie gras with leeks and truffle was a mind-blowing sensory overload of smells, taste and velvety texture. The parcels of this silky offering were covered in a truffle foam that vanished in your mouth as soon as it was tasted. These were some of the richest flavours a kitchen can offer, yet they were so delicately treated that one generous bowl was just not enough – without question, this was one of the finest dishes Diplomat has tasted in 2011. We also sampled the scallops with black pudding, crispy bacon and celeriac puree, a sublime union of perfectly seasoned surf and gamey turf.

The decadence did not subside. Having opted for a more conventional grilled turbot served on the bone with a béarnaise sauce, I could not resist the lobster and crab omelette with truffles, which came with a truffled green salad. All that truffle was by no means overkill – what could, in less capable hands, have been un peu de trop proved as subtle as it was delicious. The omelette, plump with delicate white crab meat and chunks of lobster, was topped with a perfect whole lobster claw that dared me to leave it until last.

Having gorged myself with such wickedly rich flavours, I was stunned to feel so perfectly fed and happy. Pudding was totally unnecessary, but I was not going to deny myself further gems from the kitchen. The violet crème brulé was a surprising cold pudding with crystallised violets and a peach, blackcurrant and red wine coulis, but still every inch a French classic. A selection of French cheeses was declared heroic, while the mini macaroons and chocolates that came with the coffee were so good that we were not embarrassed to ask for more.

The Capital is the stuff of legend, and should therefore be given the time to be enjoyed properly. Clear an afternoon in your calendar and indulge.