Ramen Seto, W1B 5PY
On the border between Soho and Mayfair, just across from the back door of Hamley's, is this Japanese noodle restaurant.
Basically ramen, but with a variety of other dishes available. Think of Wagamama, but without the pretentiousness and with rather better quality. Prices vary, but you can be well fed for 10.
It's often packed out, especially in the evenings - I think Time Out may have reviewed it recently.