Kulu Kulu is an old favourite of mine. The food is amazing - very fresh and a great variety - and is also brilliant value for money. There is always plenty of ginger, wasabi and green tea on hand.
If you eat there in the evenings it is wise to know that they close fairly early (around 10pm) so go around 7pm. Kulu Kulu is as close to the real thing as you will get in London, apparently, and the large number of Japanese who eat in there is testamony to this. Yo! Sushi and all of those other faux Japanese eateries just don't compare.