slippyr4 said:
I was (very briefly) quizzing my other half about this last night (she's a lawyer). She said that its very likely that you would need to settle the bill with him in full, and then take him to court for damages - rather than withholding all or part of the payment.
With the greatest respect to your missus, she would say that, wouldn't she? Cherie Blair would say the same.
Speaking as someone whose wife is a magistrate, and who knows more than a little about court proceedings himself, the judicial system does take account of individual cases. It does, by and large, look beyond the letter of the law. It is there to interpret law as well as impose it. Most county court judges would look at your case sympathetically. The fact that a registered Corgi engineer on whom you were relying has installed illegal equipment strengthens your hand considerably.
DO NOT, UNDER ANY CIRCUMSTANCES, PAY THE BILL IN FULL (forgive the caps). Your only recourse is then through the courts which, as others have advised, is not necessarily the best course of action. The guy may go bust or do a runner.
If you pay anything, it should be at most the cost of the materials he has bought. Whether this includes the boiler is debatable. He has paid out for it but it's illegal in the absence of certification which he has not supplied. Should push come to shove, I cannot believe that any judge would find against you simply because you withheld all, or part of, the cost of the boiler. He would have to have a very smart lawyer indeed to wriggle out of compensating you on that alone.
Most arbitration services work on the principle that aggrieved parties should, in the first instance, try to reach an agreement among themselves before the weight of the law comes to bear. This case is no different. If you have the stomach, which I believe you have, you must seek rectification from the guy before going to court. But don't pay him any more than you have to.
He will know how much he's skimped on the job and may be surprised that someone as canny as you
has paid him anything at all.