Ciao Bella in Lamb Conduit Street (Bloomsbury) is cheap and cheerful

http://www.ciaobellarestaurant.co.uk/reviews.asp

Bahn Mi Bay on Theobalds Road (just before where it meets Grays Inn Road) is cheap little place for vietnamese rolls, salads and rice boxes. Freshly grilled meats, perfect to eat in (gets a little smokey) or to take over to Lincolns Inn Fields (across the road) and eat in the gardens.

I also like the Hare & Tortoise , there's a few scattered around the city and do a mixture of Asian foods. Always seem to be a queue at the one in Bloomsbury (lots of Asian students locally), everything i've had there has always been good.