Where does a man go in London if he wants to eat well and conduct a little discreet business at the same time? Rumour has it that the coolest place in town is Dinings, in Marylebone. The name has a touch of Thirties country house about it. You can imagine it as the backdrop to a Noël Coward play or a John Masefield novel. I envisaged log fires and old, squashy furniture. Very English, traditional and masculine. Wrong.

Yet as soon as you walk in you sense that this is where you're meant to be. Chemistry is always impossible to define, but Dinings has that intangible 'something' at the heart of every successful restaurant - even though it breaks all the rules. It's too small and the décor is unbelievably plain. Maybe that's the secret. There's nothing to do other than stare at the person sitting across the table; nothing to distract you from wheeling and dealing except for the welcome interruption of food prepared by a Nobu-trained chef. From the outside, Dinings looks so small I was convinced it must be like Doctor Who's Tardis, deceptively concealing a spacious interior. It isn't.

The tiny upstairs sushi bar leads down a narrow staircase to the eating area which is divided into two little rooms. People often say small is beautiful but they should try being 6ft 4in and a bit. But never mind: I stooped and managed to avoid the low ceiling, and we certainly received a big welcome from a maitre d' who seemed genuinely delighted to see us as he showed us to our teeny table.

The walls are plain and there are dark leather banquettes, a bit like an anonymous waiting room. Blackboards announce the day's specials, which sounded very special indeed. It isn't often that you see tuna cheek on the menu - thank heavens, I hear you say - and I agree. I don't want to see part of the poor fish's face balefully staring up at me. However, there was plenty on the à la carte menu that did appeal - as well as a very reasonably priced set lunch - so we decided to mix and match, with some advice from the jolly maitre d'.

To begin, we shared deep-fried baby squid. These were sublime --perfectly small and coated in something crunchy and delicious - with three different sauces for dipping. We had this with incredible sugar snap pepperonchino (hot in every sense). Next came my hot donburi, which sounds like a treatment you might get at a health spa but is, in fact, a large bowl of rice topped with vegetables and meat (or fish). I chose beef, which was thinly sliced, bloody and delicious. The most extraordinary dish on the menu was the duck tataki, alone worth visiting Dinings for.

The sushi was faultless -flavoursome rice cocooning buttery-soft avocado. It's rare to so enjoy something that is so good for you.

Puddings in Japanese restaurants are so often an afterthought, added carelessly to satisfy sweettoothed customers, but with little relevance to the rest of the menu. Not so here. Thinking he'd cleverly spotted a typo or that it was intended for alpha men, the actor chose macha crème brulée. In fact, macha means green tea: the creamy sweet was a wonderful pistachio colour. My black sesame crème caramel looked like a miniature glass of Guinness and was one of the best desserts I've ever eaten. Try it.

Other customers included two chic Frenchmen, a giggling couple, plenty of interesting-looking people who were clearly conducting deals (which in the present climate was a relief) and two who were celebrating working for themselves. The place feels remarkably private and I had no sense of being overheard or overlooked. There's a certain protocol about maintaining personal space, which is unspokenly adopted here. Booking is essential.

I have only two complaints. Although the staff are completely charming, service was a little hit and miss. Also, there's a cage-like wooden structure in between the two dining rooms into which staff scurry to answer the phone that rings incessantly. In a restaurant this size, an annoying jangling ring tone is redundant.

If I lived in Marylebone, I'd be eating here every week. Even if you live on the other side of town, Dinings is definitely worth a visit. I loved it. Maybe small is the new big.