When i had overheating problems on my tddi i changed out the;
-thremostat (overheating went up from 60mph to 70mph)
-Radiator (from 70 to 80)
-cyclinder head temp sensor (overheating sorted, well until the night i killed the engine going to top speed for a few miles)

however it was still boiling and had oil in the header tank, and when i took the old engine out, there was oil in cylinder 3. turned out the pistion ring/head gasket had gone.

I'd suggest changing the Thermostat (£12) first then the cylinder temp sensor (about £30-£34 iirc)