I have to tell you I am not a motor mechanic, but my experience is similar to yours so all I can tell you is how I managed the situation.
I tried all of the things that have been suggested - thermostat, sensor, head gasket, skimmed head, pressure test, etc. Not the water pump though, because that didn't seem logical to cause my problems. I changed the radiator, but the replacement recon radiator overpressurised and burst along the plastic pipe on the top. Luckily, I was able to get a replacement free under warranty but by then I had finally accepted that the head must have a fine crack which was causing the pressure in the cooling system when the engine was hot.
I asked about the possibility of damage in the block, but nobody I talked to had ever heard of a cracked block in a diesel engine, so I found a new head and had it put on.
Interestingly, the first time it ran, the second replacement radiator started leaking through a split in the top pipe, but the mechanic felt that it was caused by an air lock because the system had not been bled properly.
So - he put the original radiator back on, and bled the system very carefully - and the result is that I took quite a few weeks before I got the courage to drive on a long journey (there is no worse feeling in the world than spending a lot of time and money on a problem, then finding that the problem is still there) but finally, having had no problems locally, I made a 200 mile round trip on the motorway with absolutely no drama whatsoever.
I still don't quite trust it, but every day that passes gives me a little more confidence - hey, don't let's get carried away here though, it is still a Land Rover and every day without problems is a blessing.
I really hope this is of use, but remember, this is only my experience and not a guaranteed solution to your situation.
Best wishes
