I phoned the bloke as soon as it happened and have been in contact every day since. Initially it looked like just the head gasket had gone, which he agreed to pay for the repair of. I took it to Pete7's Landrover mechanic "Crazy John" who seems to know his stuff. He took it all apart and discovered pistons 2 and 3 had "picked up" (what exactly does that mean?), the head I believe has warped, the gasket of course needs replacing. It would seem this is all a result of a dodgy radiator.
John has quoted me £1500 to supply and fit a replacement engine, rad, and a few other bits he thinks it needs.
As soon as I told the seller about the engine trouble he became a little less helpful, although was still polite and said he would help on the phone. I'm now struggling to get hold of him; yesterday I rang for an update and got his wife who told me he was too ill to talk to me on the phone
My current plan is to pay to get the engine changed and get the car back on the road; at the mo I'm without wheels which is a total nightmare. Going to hire a car later today. Once the repairs are done, I shall be sending a bill for repairs and car hire to the seller. I suspect he won't pay it, in which case I will take him to the small claims court and see where I get.
Also, I didn't test drive the car before I purchased it, which I believe from my legal friend works in my favour.