Within two minutes of arriving at Benja, which promises "a new type of Thai cuisine" based on "the flavours and tastes of Royal Siam", it was to the inspirational advice (expirational advice, technically, but we'll let that pedantry pass) of a certain Anna Leonowens that I found myself turning. "Whenever I feel afraid, I hold my head erect," Missus Anna taught us in song, as she struggled to acclimatise to life with Yul Brynner, "And whistle a happy tune/So no one will suspect I'm afraid."

But I was afraid, as a nervous looking staff clearly suspected, and even a whistled chorus of an exceedingly happy tune (Ossie's Dream, the Spurs' 1981 Cup Final anthem) couldn't dissipate the fear that this was a wildly misconceived project ... a restaurant with wildly grandiose ambitions fatally undermined by the premises.

Fortunes have been spent on fitting out three floors of a house in Soho with slate grey walls bedecked with lurid ceramic carp, two-tiered ceilings, intricately patterned silk banquettes, gilded wall panelling and, in the ground floor room where we ate, a stone statue of a Siamese maiden in an alcove all her own.

It's sumptuously designed, but that sumptuousness demands a good deal of space and light to set it off. What it gets is a pokey town house better suited to a bustling trattoria, and the effect of so much lavishness in such a small, narrow, underlit room with tables crammed together is assonant, gloomy and oppressive. A glance at an inadequate wine list and a brief, unenticing menu brought the lips together once again, this time to dip into the oeuvre of The Clash (Should I Stay Or Should I Go).

According to the publicity, the head chef is one ML Kwantip, scion of a family that for centuries cooked for Siamese kings, previously "guest chef" herself at a major Bangkok hotel, and a columnist for Thailand's leading woman's magazine. Whether "head chef" is an honorific title, allowing her to dictate menu and recipes from Bangkok, I've no idea. But if she was in the kitchen this lunchtime, and if she cooks like her ancestors, you can see why Brynner's king was in such a foul mood.

Where chicken satay ties in with "a new type of Thai cuisine" is one for the All Soul's high table, but it lacked any succulence and came with a feckless peanut sauce. In a blind tasting, I'd have guessed "locusts" for the deep-fried frogs' legs served with crispy lemongrass, kaffir lime leaves and some much-needed chilli sauce. As for the minced chicken curry puffs, I'm still suffering stoically from the burn inflicted upon the soft palate when a bite into some clumping pastry unleashed a stream of molten chicken with a flavour reminiscent of the curry sauce offered as an accompaniment to chips in Chinese takeaways.

Two salads were dismal little affairs, "salad" here meaning "a couple of almost fresh radicchio leaves". My sister's one contrived to mingle three slices of cheap smoked salmon with a "spicy sauce" to replicate precisely - and this takes some doing - the flavour of a Polish kabanos sausage. My mushroom salad "north-east style" (laced with red onion and fresh chilli) can be dismissed with a brusque etcetera, etcetera, etcetera.

The main courses were little better. A potent sauce of basil, chilli and garlic couldn't disguise the poor quality of a piece of allegedly sirloin steak, and while the initial flavour of my stewed lamb in yellow curry was impressive, the aftertaste was unsettlingly and bitterly cloying. The same went for a lukewarm medley of stir-fried vegetables in a thick, soy-based gravy.

"You will be nice about the staff, won't you?" said my sister as we settled up, and I certainly will. The service was friendly and sweet, notably from one of the most delightful waitresses I've ever met. She even came close to letting us smoke in defiance of house policy. "In July, no smoking anyway by law," she pointed out, but I couldn't help wondering whether Benja will live to see the day. "What's that you're whistling?" asked my sister. "A happy tune called Fauré's Requiem," I said, and after receiving a Buddhist bow from the waitress at the door, we took our leave.

Rating 4/10

Telephone 020-7287 0555.
Address 17 Beak Street, London W1.
Open Mon-Sat, lunch noon-3pm, dinner 6-10.45pm.
Price À la carte with wine, £45-£55 a head.