I've just come across this site which has treasure hunts already done for a variety of locations in the UK - mostly larger towns, but not exclusively (Birmingham, say, v Ambleside), and it says you don't even need specific local knowledge for them (although if you read the child's comment on the Norwich one you might quibble )
huntfun
You can buy them locally or online and they also give helpful hints with organising. Seems like a really good idea!