The Belgian Monk (mentioned above) is nice but rather over-priced.
If you happen to be in Norwich in the
evening (Wed to Sat), the Golden Star's food is excellent (and it's a really nice pub):
http://www.goldenstarnorwich.co.uk/
The lunchtime menu is far more basic (but still tasty)
If you enjoy Thai food though, the best place for lunch is The Vine. It's the smallest pub in the city, with an equally small restaurant upstairs but it's an absolute delight and the food is great:
http://vinethai.co.uk/
(It's easy to find. Look for the Tesco on the edge of the market square and take the path that runs down the side of it. The Vine is just a short way along, on the right)