Thanks for the suggestions.
I replaced the old engine (also a disco 200tdi) a few months ago and it is only since then that the problems have started, that's why I didn't think it would be the radiator or temperature gauge. The old engine never used to overheat with the same radiator, also surely on the motorway is when the radiator would be most effective???
Is it possible that I could have damaged the gearbox when I put in the new engine? When I started the car for the first time the clutch didn't seem to dissengage at all but when I tried again it was fine. Could the problem be the clutch not fully dissengaging? but it seems to be fine selecting first when stationary.
I also recently replaced the gear oil. It was definitely the correct oil and I filled it correctly to the level of the filler plug....
Thanks for the help, I'm running out of ideas!