You know that consumer magazine “Which?” I remember they used to have a legal advise spot on the back of the magazine, this was waaaay back in the day, but I have always remembered one bit of advice from their legal department .
A guy bought a Jaguar second hand with a full MOT, a few days or weeks or couple months, the rear axle collapsed on the road. The legal advise was that there was no legal come back on the seller, maybe at that time the regulations were not strict as they are now with selling an unroadworthy vehicle. But there was a case to answer for the MOT testing station.
Because severe rust cannot happen overnight nor can it happen within a couple of months. They obtained an expert report the rust was severe and could not have been missed by a competant MOT tester. The MOT testing station was sued for the full cost of repair to the vehicle.
Your problem will be if the rust could be passed by some testers and failed by others. You might want to talk to your local VOSA test and appeals station to see if they can offer a report writing service, which you can then take back to the MOT tester and your seller, because today there are laws against selling cars which are not roadworthy.
You have to act fast on this to get a report done. not going to be easy with Covid19.
Your first step should be to speak to the seller to see if they will offer anything without a report.
Depending on how much you paid, you could end up best off speaking to a solicitor.