Please contact your local authority to see if they have a 'confidence list' of builders. Or try Trust Mark's website. When you get quotes in, make sure you ask to see work previously done, ie, do a site visit yourself, of work done by the builder, and make sure that you are absolutely sure they know what they are doing.
Warning to everyone out there. The law sucks in this country when it comes to building work. If your builder does a bodge job and he is dodgy, if you've already paid them, no-one will be able to get your money back for you, not even the best lawyer in the world. Here's why, if the builder has a limited company he will simply dissolve his company and claim to be bankrupt. Even if you get court orders against him to repay you, if he ain't got the money, you won't get anything.
You can report them to Trading Standards but all they say is to give them two more chances to fix the work and then you can sack them. But they won't go after the dodgy builder - you have to. At your expense. Only if TS has loads of complaints about the same builder, will they step in. I think.
Even if a builder belongs to a trade association, these people say they will step in but the onus will be on you to show that the builder's been dodgy and then you may be lucky if you get your money back or any redress. Find out for yourself how these different trade associations admit someone for membership - it's simply a case of paying the association for your membership fee and you'll be in. They (the trade association) don't necessarily check out the builder's credentials.
Only use a builder whom you've checked out and checked out and even if they have indemnity insurance, don't be so sure that you will be able to claim on it - yes, that's right. The builder has to agree that he has bodged up your home/the job and he has the authority to give his insurance company the go-ahead on the claim! So you are stuffed if your builder does not want to admit liability.
All in all, a very scary prospect so you need to find a builder who is honest, reputable and won't do a dirty on you when things go wrong. 'Cos if you have problems, you'll be on your own baby. Not unless 'cowboy builders', the TV programme, can come to your rescue... because that, it seems, is your only hope when things go badly wrong. All because your builder turned out to be dodgy...