Used car mis-described. What recourse do I have?

[Hammerhead1914]

Hi all, I bought a modified Ford RS. The trader who sold it to me works for as/a garage and had good reviews. The sale has gone through a private bank account via BACS. It's apparently sold as seen but I was issued no receipt or paperwork except for a V5 doc that was still registered to a previous owner, not the guy who sold it me.
The garage the trader is associated with has issued a brand new MOT on the vehicle. A bad knocking noise that wasn't apparent when I bought it developed within a day. A garage I have taken it to have stated incorrect fitting of suspension system. Also that there is no CAT on the exhaust. They have stated that they have sufficient concerns regarding the quality of modification works on the vehicle, that they are refusing to carry out any works on the vehicle
Seller is now non-responsive and I have a defective vehicle I have paid a lot of money for that I can't really drive until I get expensive works done on faults that would have been apparent when this alleged MOT was carried out.
Any advise on how to proceed would be greatly appreciated. I feel overwhelmed with the potential for spiralling costs which in my opinion the seller should be liable for. This vehicle was described in the advert as in superb condition mechanically.
Thanks in advance - I'm learning as I'm going.

 

[Hammerhead1914]

So I think I have overstated when using the term "garage". It's a private trader in reality. Unfortunately I don't have the hard copy of the MOT certificate, only what is accessible online which gives a test no. but doesn't confirm the garage that carried it out. I've definitely dropped the ball on this, hands up. I'm just trying to establish what if any recourse I might have to recover costs. And also if anyone has any recommended specific checks on sellers to carry out to avoid this going forward. Thanks again.