Overheating

I have a Defender 2.3 Turbo Diesel which runs very hot on the motorway (almost on red on the temp guage). Consequently the heater produces red hot air after about 20 mins at 60mph. As the summer approaches this is not ideal. Any ideas on a solution?
salgray

Have you checked the fluid level?

Does the pipe from the engine to the rad (top pipe) get warm - if not the thermostat might be jammed shut.

Water pump making any funny noises, belt OK, impeller blades could be bady rusted away.

Could be a blockage in the rad, if its hot at the top is it warm at the bottom indicating it is cooling the water ok.

Or could be an engine issue.

Your injecton timing could be out.