Which Child Seat

[Hackney]

Having hired a bike with a seat in my local country park I'm thinking of getting a child seat (and *cough* a new bike *cough*) but have no idea where to start. Halfords has about 30 ranging from £30 to £150+ from memory.

What are the good brands?
What are the features to look for?
Do they tend to fit *any* bike - I'll probably get some kind of hybrid