Cheers for the help. I bled it with this method this evening, took it for a drive - problem is still there when turned off!?
It doesn't go above 82 degrees whilst driving, even when giving some boot.
I am thinking that it might be corrosion in the cooling system? The radiator is clean as a whistle but inside the radiator pipes on motor side doesn't look too healthy. I have some cooling system flush on the way so will give this a whirl and see how i go.
I suppose the only other thing could be air getting into the system somewhere... in another thread someone mentioned the turbo water supply/return hoses.
Could it be anything else?