This is a terrific place, a small chain of restaurants with 4 branches in central London.
Reservations aren't possible, you just turn up. For this reason, it's better to go about 7pm, later and you might have to queue for a while to get in.
The restaurant is in a basement, decorated in cool whites and creams, with pine tables and benches. The tables are in long lines, with benches running along them, so it's not a good place for a private chat, as there will be people on both sides of you able to hear!
The food is essentially post-modern Japanese, no sushi, and all pretty healthy. My favourite thing there is a meal combination, called "Absolute Wagamama", where you get chicken, greens, and noodles in chicken stock dish, accompanied by a side dish and a glass of their amazing fresh, pulpy fruit and vegetable juices. They do many other dishes. Other favourites of mine are the fish cakes, and the spicy beef noodles. As well as the amazing fruit juices, they do a selection of Japanese beers, and fragrant teas.
It's a great place, with a good atmosphere, and I heartily recommend it. The whole restaurant is non-smoking.
To get here, take the tube to Tottenham Court Road (Northern and Central Lines), the walk about 20 yards north up Tottenham Court Road, turn right into Great Russell Street, second right, then first left. It's about 250 yards altogether from the tube station.