Ok haven't read all of the posts but a few points from me.
The NiC will tell you you cannot withhold Certs, and they are right!, the regs state that installations must be inspected, tested and certified upon completion, and withholding the Certs contravenes this!.
So the OP's situation.
1, document a written account of what has gone on in chronological order, including what has been agreed, done, said and altered, and more importantly by whom.
2, if you can, take pics, but don't worry if you can't, as they don't prove who is responsible for the bodgery.
3, report to LABC and you scheme
4, copy the correspondence to the client with a letter explaining that the installation is unsafe and does not comply, and that it should not be energised.
6, add to this correspondence a invoice for works completed as agreed by them at the rate agreed by them. Explain that they have 7 days to pay in full, after this time the value attracts the statatory 8% that you are legally allowed to levy, advise them that after 14 days it will be passed for collection. Refer them to your account of proceedings if they have any issues with this.
7, after 14 days issue a county court claim form( free and easy to do)
Always remember that the responsibility for anything electrical lies with the last person who touched it.
Sounds like relations are poor so you have nothing to loose by getting hard and serious with them!
Just Claim :: N1 Claim Form