A lot depends on what you are going to be using them for - serious hill walking or just country rambles - how often, and in what conditions. And also your budget - I wouldn't suggest scrimping on something that you may be spending many hours in day after day.
I agree with what DuncanH says above - make sure you try before you buy, wearing the type of socks you would wear while walking and also try them on an incline (decent shops have a ramp for this) to make sure your foot isn't sliding around inside (=blisters).
Once you've tried on and found the perfect fit, don't forget to go online and find the best price
On brands, I've got a couple(winter/summer) of pairs of Scarpa which I love - Beghaus, Meindl, Mammut, Grisport, La Sportiva will all do a good job for you if they give you the fit you need.
And avoid Karrimor or anything else you can get at Sports Direct!