Is the new thermostat OEM or aftermarket? I fitted a britpart one and had to replace it in the end for a BMW one. (I have used lots of Britpart stuff without problem) Also, the expansion tank pressure cap might be worth checking.
Might you have an airlock? these are very difficult to avoid airlocks while refilling coolant. Best to set the heating to maximum and run the engine from the beginning of refilling the cooling system.

Energy8 thanks for the reply, i wish I had seen it earlier!! in between posts and on the advice of my mechanic (not the one fitting) I plumped for a new cylinder head fitting - all good for three weeks then the radiator blew this week.

I've just fitted a new one radiator this morning and same overheating problem. funny enough i was discussing with my brother in law a few weeks ago (ex REME for 22 years) and he suggested it was not the cylinder head but the thermostat, yes it was a Britpart. i'm going to drain and refil to make sure i have no airlocks - after reading various articles, it is possible i didnt do it right, failing that i'll try a new thermostat.

thanks for the advice and have a great new year