This can be a very stressful time for you, and from all the evidence you have supplied so far you are not in the wrong. We can all give advice but we are only electricians.
If you contact Andy he will be able to inspect the works carried out and give an up to date certificate on the condition of the wiring. You must get in touch with a solicitor, the first appointment could be free, and you will need expert advise.
The certificate of conformity is worth nothing, and he has broken more regulations than you could shake a cat at.
Keep all paperwork, and log any calls this builder makes and try to get down his exact wording during any of these calls. You yourself could be in a legal position of breaking the regulations, this is because the Part P of the building regulations place the responsibility with the home owner, however I would not worry about this, you probably thought that the work being carried out was to all standards and regulation.
Before you report him to anyone get a proper electrician (Andy) to access the work, when you have a copy of the report contact a solicitor. Any costs in putting the work right could be claimed via damages from this builder including any outlay you have had.
When you have seen a solicitor contact your local Building Control, Trading Standards, and give that nice bloke a call from that TV show Rogue Traders, and come back and let us know when the program is going to be shown
