To access the top-floor restaurant in London's top fashion department store, you must travel up five escalators, past mannequins of increasing barminess. The head of the ground-floor mannequin is made up of nuts and bolts, the next is constructed from cocktail bar paraphernalia, the next from kitchen cleaning products ... it's all so determinedly wacky, you wonder what shrine to surrealism awaits you on the top floor. The big surprise is how un-surreal it is, how studiedly old-fashioned.

The room is a long oval, pale and warily tasteful as if afraid of upsetting anyone. The walls are a discreet grey with the kind of scribbly design that you see at Heathrow when you're on the way to the departure gate. A certain drama is provided by the paintings, vivid close-up portraits of beautiful women, their sad, downcast faces made up, in pointilliste style, from thousands of golden nails. The floor is shiny modern parquet. The napery is virginal white. The chairs are lightly toasted beige. The ceiling is an astonishing lattice of translucent glass tiles that make you feel as though you're sitting inside a Zeppelin. But there's something dispiriting about the dining areas that are set back from the main room: they're dark but not romantic, as though the lunchers have been put in detention.

The reason for the muted colour scheme is, of course, to let the clientele show off their finery. Predictably, the fashionistas at the next table – a middle-aged blonde and two dark-haired young women, possibly her stepdaughters – are all clad in Stygian hues, the former in navy-blue jumper and skirt, the latter in skin-tight black. And all are sporting black-leather jackboots, as though it's a uniform. One of the younger women is accessorised by a baby, decidedly off-trend here in a red romper suit. When she cries, her mother drapes a shawl with a Mexican-skull pattern over the Maclaren Dreamer buggy. It doesn't shut the child up, but it looks fabulous.

Anyway, about the food. The Fifth Floor Restaurant stands out from its downscale sisters, the Champagne Bar, the Café and Roof Terrace, by its serious menu and top-drawer prices. The à la carte menu offers two courses for £35, three for £40, but there's a less heart-stopping "Market menu" at £19.50 for two courses, the ingredients selected from Harvey Nicks's own Fifth Floor vegetable shop. I was lunching my son Max – it was his last proper meal before he starts a year of Pot Noodles at Sussex University – and explained the virtue of restraint in ordering food. Naturally he headed straight for the à la carte and wouldn't budge.

How does one describe the food at Harvey Nicks? Very pretty, very svelte and very anxious not to appal or upset gentle palates with rough textures or butch flavours. An amuse-bouche of chickpea soup with golden raisins and olive oil was horribly misconceived. ("It's too sweet for soup," said Max, "and too creamy. Like Ambrosia Creamed Rice." I completely agreed.) My smoked-salmon starter was marinated in elderflower and served with fennel and apple crème fraiche, plus three tiny sections of orange. A beautiful sight, and the fennel-apple combo complemented the salmon just fine – but I longed for a wedge of lemon to bring some excitement into its life. Max's roast scallops with braised chicken wings promised a hearty wallop of flavour; the tiny pieces of boneless chicken were delicious, and the scallops, barely touched by an oven, were given a tiny crunch – the politest possible crunch – by the molecules of hazelnut in the jus.

It was all perfectly agreeable. The home-selection French sauvignon was fine. The waiter, handsome as a Paul Smith runway model, smiled characterlessly. Some yards away, five ladies of mixed race greeted each other with silent air kisses. The baby under the Mexican-skull shawl slumbered peacefully. Nothing could disturb the frictionless blandness of the Fifth Floor. Heaven, the top floor of everything, must be a bit like this, without any sense of edge or commotion or the possibility of anything going wrong.

My roast saltmarsh lamb rump, five pink roundels of meat so pretty you could give them to your sister as earrings, sat on a hill of lightly spiced chickpeas flecked with red and green traces of spinach and pepper. It was a plate of costume jewellery, perfectly served, unearthly on the tongue and wholly forgettable. Max's pork belly with black pudding (how did that get on the menu?) offered a more serious array of autumnal colours, but worked at all levels: the pork immaculately cooked, the layer of belly fat succulent, the black pudding soft and all-pervasive, the caramelised apple and sauerkraut on a little pastry raft a yummy addition to the flavour catwalk.

We shared a fine vanilla pannacotta in a cocktail glass with apple purée and blackberry jelly, finished our glasses of the house Merlot, and Max departed for student life. I was glad to give him this glimpse of the high life; and to share the discovery that the height of sophistication can sometimes taste of nothing at all.

Fifth Floor, Harvey Nichols, 109-125 Knightsbridge, London SW1 (020-7235 5250)

Food 3 stars
Ambience 2 stars
Service 3 stars

About £120 for two with wine

Tipping policy: "Service charge is 12.5 per cent discretionary, of which 100 per cent goes to the staff; all tips go to the staff”