Having taken the engine apart (the rest is coming out this afternoon) it would seem the radiator was probably the cause of my troubles. As you can see from the photo it's in very poor shape, and had a lot of crap on the front of it. The other one shows the damage that was caused when the whole thing got a bit warm.
Replacement engine bought, all going in next week with new rad etc. I'll post an update regarding the situation with the seller once it's all done and dusted.
If anyone in Hampshire needs a good landrover mechanic, Crazy John is your man. Pete7 recommended me to him and he really knows his stuff. He is also very reasonable when it comes to how much he charges.