I agree with Mike & Snowhead, the dispute is between yourself and the seller, this is clear.
Having said that the electrical issues seem minor and would only take a couple of hours to correct, so if you allow a day and some materials you are looking at £300 or less to get a safe installation.
With that in mind, I’d instruct my solicitor to write to the seller a letter explaining that he/she has committed fraud, outline why and demand say £500 to correct the defects to stop further proceedings. I don’t think I’d take it any further than that due to cost, but I think you would more than likely get a cheque through in a few days.
Looking at the certificate I don’t think that you have a particularly dangerous installation, the test results seem okay and you have a newish metal consumer unit, which is probably the addition/alteration that’s 1 year old. Might be worth asking for that certificate but I’m sure if the seller had one they'd have supplied it.