Marianne is tiny and there's no bad table - unless you get a bad neighbour

Marianne, 104 Chepstow Road, London, W2 (020-3675 7750)

Not many chefs preparing to open a new restaurant are able to take their entire brigade on a team-bonding outing to Le Gavroche. But that's what Marianne Lumb did this summer. Generous, certainly; but then it does help when your team consists of just three people – you, the maître d' and the kitchen assistant.

Marianne, as the statuesque chef's new Notting Hill venture is called, is tiny – a nano-restaurant consisting of just 14 covers. On the plus side, there's no bad table. On the minus side, you might get a bad neighbour: the ambience very much depends on whether the people you are sharing the 25 sq ft dining room with are pleasant. And as the restaurant lies in primo hedge fund territory, that is by no means a given.

The thinking behind this bijou venture seems to be to extend the exclusive, intimate feel of private dining to a (somewhat) wider audience. Before reaching the finals of 2009's MasterChef: The Professionals, Lumb worked for many years as a private chef – in the "Elton doesn't do dairy" sense – and is comfortable working at that scale.

Given that there are only four tables in the place, I was amazed not to be asked for credit card details when booking. One table of no-shows would be even more damaging here than in a regular-size restaurant. But that first user-friendly interaction set the tone for what was a surprisingly relaxed and convivial lunch, under the circumstances.

The dining room, hidden behind a discreet curtained storefront round the corner from legendary gastropub The Cow, feels something like uncovering a pop-up: secretive and rather fun. But the décor is pure fine-dining: creamy pastel tones, crisp white tablecloths, silk upholstered chairs and a single wild flower on each table. The result is not quite like anything else around; it's a new kind of venture – a posh-up.

Lumb herself is visible in the kitchen– or at least a cross-section of her from chest to hips – through a letterbox-shaped window. It's like watching MasterChef on a TV tuned to the wrong ratio. But on our lunchtime visit, she stayed in the kitchen, leaving crowd control to her maître d', a tactful and knowledgable chap, ex of Medlar.

The dishes on Lumb's short menu owe nothing to prevailing food trends, and everything to quiet good taste and premium ingredients. Between three of us, we ate everything on offer, a series of refined and perfectly-worked dishes which also delivered some big flavours. A a warm starter salad of red-legged partridge with lentils was gutsy yet delicate, as was a scallop dish anchored by artichoke purée and the salty slap of pata negra.

Fillet of Wagyu beef, served pink (and costing an extra £10), came with nothing more than its own juices, and a perfect, swoony gratin dauphinois. Both it and a sumptuous slab of poached turbot were garnished with a seemingly unseasonal veg, which the menu hastened to explain was Wye Valley autumn asparagus – a new one on me.

Occasionally the refinement tipped over into Gordon Ramsay-esque Frenchified politeness. A veloute of autumn squash, bright and light, with its bursty little burrata-stuffed tortellini, needed a bit more bass to balance the treble. And to call a dish 'Mr Pickwick's boiled dinner' when it is a limpid broth holding nuggets of braised chicken and beef fillet, a pot-au-feu in all but name, is a crime to enrage at least one of the MasterChef judges.

The wine list, which, like the Notting Hill housing market, ranges from pricey to prohibitive, also offers a few attractive entry-level options. There's nothing by the glass, but our butler, sorry, the maître d', offered to open anything for us. But I think by then he had rumbled me as a critic. It's hard to stay anonymous when you're on such public display, and when one of your guests (that's you, Emma Freud) seems to be loudly auditioning for a judging role on the next series.

Once our neighbours had left (ie, were driven out), the intimate, beautiful little room came into its own, a convivial cocoon of talk and food and fun. That dinner-party vibe was reinforced by the appearance for dessert of every Notting Hill hostess's favourite, chocolate nemesis, a lighter-than-usual version strewn with tiny flowers and served with figs and damson ice-cream.

The cheese board included a brie truffled on the premises, which pretty much sums up Lumb's attention to detail. Her food isn't grabby, or agenda-setting, but it's terrific, and there's a novelty factor to Marianne which could well make it A Thing for wealthy west London types. Probably not somewhere to go if you dine out for the theatre of the experience. But certainly worth a visit if you're happy being centre-stage yourself.

Food ****
Ambience: see above
Service *****

Marianne 104 Chepstow Road, London W2 (020-3675 7750)

Lunch: £38 for three courses. Dinner: £48 for three courses, £60 for four courses, plus wine