Cambio de Tercio, 163 Old Brompton Road, London SW5

In bullfighting it means to dramatically change your approach. Which may strike you as incongruous given the staid, conifer-lined streets around Old Brompton Road are roughly as far from the horns and matadors of Pamplona as the Horsehead Nebula is from Hogwarts. And yet, superb food apart, the appeal of Cambio de Tercio is largely owed to the unlikeliness of its location. It is Spanish and sexy, in an area of south-west London that's English and awkward.

Two sights on the short walk from Gloucester Road Tube station have stayed with me. First, we pass a man in a pastel-coloured V-neck practising his golf swing in the street. He is holding a club in his hand, the boot of his Lexus is open, and do you know, after completing each stroke, he holds the pose and smiles in satisfaction. About 100 metres on, a wealthy family emerge from a trattoria, stinking of old money. The kids, my girlfriend says, "are dressed like bad parodies of their parents", all Barbours and salmon-pink corduroys.

If one wishes to become rich, Oliver Goldsmith said, one must appear rich. This part of town can feel like a giant ceremony in ostentation. And the thing about ostentation is it is usually a cover for lack of substance, for inauthenticity. But you can't do more substantive or authentic than the ground-floor room in this restaurant, favourite haunt of Rafa Nadal, who eats here every night during Queen's, the Wimbledon warm-up event.

It's crammed, but cosy. The mustard-yellow, black and terracotta walls fuse into a thickly Iberian setting, with Spanish art of the 20th century giving life to the walls. An incessant supply of hugely pregnant women and their wallet-wielding partners imbue the air with joyful anticipation; and they are aided in their effort by the magnificent owner of this joint, Abel Lusa.

Oh, Abel. He set this place up with David Rivero in 1995. Emerald-eyed under dark eyebrows, a monument to Latinate lasciviousness, and dear compadre of Señor Rafa, he is hard to resist – so when the new tasting menu is recommended, we take it.

It disappoints. Not because, at £37 a head, it's especially poor value for money. Rather, it stops just short of being outstanding, spoilt by two weak courses in the middle. The young Spaniards who wait the tables so stylishly greet us with a lollipop of Manchego cheese. Grated so as to add air to the crunchy bite, it is magnificently intense, a clever opener of tastebuds. Then comes a foie gras emulsion with yoghurt and date purée, in a cocktail dish, with roasted sweetcorn on top. The yoghurt is an inoffensive distraction. The real charm is in the intensity of the corn flavour against the foamy wonder of the foie gras.

A seared tuna salad arrives, replete with Amontillado vinaigrette on a rocket and almond salad, with peach garnish. It's a decent idea, but poorly executed: the tuna is marginally overcooked, and the peaches are raw and lack flavour. Then comes a shocking white asparagus in "salsa verde", with little torrenzos of ham to apologise for that which they're garnishing. The asparagus, which seems to have been par-boiled for six years, is flavourless and drowning in an ugly white-wine goo.

But these courses are curious anomalies. Expectations dimmed, an excellent sea bream arrives, grilled to a crunchy skin but muscular beneath, and decorated with smoked leeks, creamed fennel and the most flavoursome segments of orange I have come across – their tang preparation for an outstanding meat dish. Fillet of venison, it is, scarlet but not under-cooked, succulent and with that delicious mustiness unique to this animal. It comes with a sweet potato purée and a delicately spiced apple that make the whole thing sing on the roof of the mouth.

It calls for a sweet aftermath, so Abel gives us a lesson in sherry, which makes the prospect of re-entering the rainy October Saturday outside infinitely more bearable. So too does a torrija bread pudding, brought alive by proper use of vanilla pods and chocolate truffles of just about perfect resistance to bite.

All told, it's a hell of a way to escape the tyranny of the Old Brompton Road, and if they can only sort out those two dud dishes, there'll be no serious argument with the proposition that Cambio de Tercio is the best Spanish restaurant in London.

7/10

Scores: 1-3 stay home and cook, 4 needs help, 5 does the job, 6 flashes of promise, 7 good, 8 special, can't wait to go back, 9-10 as good as it gets

Cambio de Tercio, 163 Old Brompton Road, London SW5, tel: 020 7244 8970

Lunch and dinner, daily. Dinner for two about £140 with wine

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