Eating in Bruges

We are meeting some friends from England in Bruges on Saturday and we need to find somewhere for some lunch. I'm not that familiar with Bruges, but from previous experience know that eating out can be very pricey.

Does anyone know of anywhere fairly central for Saturday lunch - not even a big meal, just something cheap and cheerful? Also, one of us is vegetarian - any ideas anyone??

Happy New Year to you all and thanks for any help.