Isarn, London N1

Wagamama's little sister

To that list, we can now add another high-achieving pair of siblings. I hope Tina Juengsoongneum, the co-owner of this week's restaurant, will forgive me for mentioning so early on that her brother is Alan Yau, the genius behind Wagamama, Busaba Eathai and London's finest Chinese restaurant, Hakkasan. After all, the connection seems to be working for her. Despite being a smallish, family-run Thai restaurant in a not-very-lovely stretch of Islington's restaurant mile, her new venture Isarn is already packed out, and starting to get glowing reviews. And that's without any publicity.

The Yaus grew up in King's Lynn where their parents ran a Chinese restaurant. Tina later worked alongside Alan at Wagamama, and then at Busaba. For her first independent project, she and her Thai husband Krish have chosen an area of London not yet colonised by elder bro. In the mêlée outside, Isarn's discreet frontage strikes a distinctly reticent note. Inside, the long, narrow room is sexily decked out in black wood, and floating high in the eaves are extravagantly huge lampshades, their butterfly cut-out motif echoed in the miniature replicas that are suspended above each table at eye level.

The suspicion that this is no ordinary high-street Thai is confirmed by the charming lady who materialises within seconds to show you to your table. Only if you've booked, mind. At weekends, Isarn is already turning customers away - the locals who accompanied me were still smarting from rejection the previous night.

The menu offers the final confirmation that here is something a bit special. Some of the dishes - the pad thais and green curries - are standard Thai restaurant fare. Others, such as monkfish curry, or seared tuna salad with chilli passion fruit dressing, are directional, pan-Asian hybrids along the lines of Busaba Eathai.

Our meal got off to a fantastic start, with a crisp, golden tangle of soft-shell crab, scattered with fried garlic, chilli and coriander and dressed with fresh green mango. It was so good that we immediately ordered a second serving.

Of what followed, the simpler, cheaper dishes tended to be best. We particularly enjoyed a delicate mound of steamed savoury rice, depthcharged with holy basil and chilli, and partnered with succulent slices of roast chicken, and a perfectly spiced red curry made with plump grilled prawns. A stir-fry in which spicy chicken was tempered by pulpy green aubergine was also a great success. Less impressive were two of the more expensive dishes on the menu, a slightly desiccated seared lobster tail in tamarind sauce, and a coconut broth brimming with tiny baby clams (both £11.50), which didn't repay the hard work involved in de-shelling them. Still, our fault for ordering them.

The food is beautifully presented, on weirdly shaped tableware fired in jewel-like colours, like Denby Ware reimagined by Salvador Dalí. Equally beautiful were our fellow diners, who seemed to glow in the all-black room. Maybe it was just clever lighting, but everyone else in the place seemed to be in their twenties, so much so that Harry anxiously, hissed, "Why's everyone so young in here? It's like Logan's Run!"

The back of the room opens into a little yard, which could be a nice place to eat on a fine summer's evening. Until, that is, the curtain falls at the King's Head theatre pub next door. At interval-time, the audience spilled out into the garden next door, their gratitude so intense at having survived the first half of the Blunkett play Who's the Daddy? that their bellows of relief drove several of Isarn's patio diners indoors.

A fight almost broke out at our own table, so delicious was one of our puddings, a Thai delicacy called "tago with taro and lotus seed". I've absolutely no idea what was in it - it consisted of cubes of green jelly topped with a slightly salty cream - but it was great. It brought our bill to around £30 a head before wine - a £26 bottle of Hunter's Gewürztraminer that survived all the intense flavours that were thrown at it.

So it's a little more expensive than Busaba Eathai, but without wishing to provoke late-onset sibling rivalry, I'd say the food was generally better. The service certainly is; our prompt and efficient waiter had a distinctly human touch, raising an eyebrow when one of us requested a helping of morning glory (it's a Thai vegetable, actually), pre-empting the inevitable phnar-phnaring.

As we emerged from Isarn into a street heaving with cafés and restaurants, our locally based friends were saying, "At last, somewhere to eat around here!" Right now, Isarn is just a very good neighbourhood restaurant. But if she has any of her brother's entrepreneurial spirit, Tina Juengsoongneum has a potential mega-hit on her hands.

Isarn, 119 Upper Street, London N1, 020-7424 5153

Food ****
Ambience ***
Service ****

Meal for two, about £60, without wine

SIDE ORDERS: THAI TREATS

By Caroline Stacey

Busaba Eathai

The third and latest branch of Alan Yau's inspired Thai diners: sultry, designery, smoke-free. No bookings, so get here fast before word gets out and there's a queue for dishes and glamour that belie the economy prices.

Bird Street, London W1 (020-7518 8080)

Dusit

Festival-goers have this highly rated Thai in their sights. Extensive menu; excellent cooking; exquisite setting. It's a superb all-rounder, so despite the prices, Dusit's seats are much sought-after. 49a Thistle St, Edinburgh (0131-220 6846)

Pacific

Chinese rules the roost in this corner of Manchester. But Pacific is a venue of two halves, and the sleek second floor is devoted to Thai. Goes well beyond green curry for a sophisticated crowd.

58-60 George Street, Manchester (0161-228 6668)

Surin

Enterprising chef and owner Damrong Garbutt is from north-east Thailand but marries her cooking with Kent's strengths. Locally caught lemon sole is a speciality; her beer is brewed specially.

30 Harbour Street, Ramsgate, Kent (01843 592001)