Through my bitter personal experience, I wouldn’t take on any builder unless I have personal references from clients and see the work with my own eyes. The temptation is always to rush in and get the job over with, but far better to take your time and do it right the first time.
A new "Consumer Right Bill" is currently making its passage through Parliament
http://discuss.bis.gov.uk/consumerrightsbill/. We can’t know when it will come into effect but it’s meant to improve consumer rights when you purchase goods/services. If you can wait for the bill to come into effect, it should put you in a stronger position for any contingency.