Never, ever, EVER use someone you've picked straight out of the Yellow Pages. Or worse, someone who's put a flyer through the door.
Good workmen do not advertise - they are recommended by word of mouth and have more work than they can handle.
Ratedpeople is a good site. So is mybuilder.co.uk. So is Problemsolved.
The best way to find a workman: whenever you see any of your friends with a beautiful new kitchen/bathroom/redecoration job, ask them the details of the company that carried out the work, and also whether they were happy during works. (A good job at the end of the day doesn't always mean a happy experience during the process). Then keep their contact details in your address book until you need them.
Or have a look round to see if you can see any houses with a builder or decorator's sign up. Ideally, look for a while, and you'll start seeing that the same firms appear again and again - this is a good sign. Then knock on the door of the houses and ask if they'd recommend them.
Someone a while ago posted about how there are a bunch of fake reviews (or "reviews" from mates) on those sites so please take them with a grain of salt and go to visit the houses of references to check up!