I've had exactly the same thing recently, but from the other point of view.
I'm a trader, I sold a 2.6 MV6...it was fine when it left me, we went out on a test drive, he bought it, he drove it home no problem. Then the next day he phoned to tell me the oil light had come on and he had continued to drive it. Subsequently discovered the engine is now seized bacause he contiunued to drive it.
I gave him a refund, and extra...so I was out of pocket, but I felt it the reasonable thing to do.
There are different ways to look at it, he had plenty of time to look over the vehicle, he test drove it. It was sold without warranty after trial. Technically it was tough for him, but you could argue that one would reasonably expect the vehicle to last longer.
What you need to establish is did the seller know about the defect? I didn't know of any issue with the MV6 I sold, it was driving perfectly and the engine was great. But, if you can prove the seller knew of anything wrong with it, that's where you can get something done.
Where was it purchased from? Private sale, trader, franchise (which is doubtful)?