Thanks for all the input.

Checked this morning and the top rad hodse is still nice and pliable, so it doesn't seem that compression gases are getting into the coolant system.

The coolant level (cold) was absolutely normal, about half an inch from top of coolant tank, so I am not losing any coolant. Rad was replaced two years ago BTW.

I will try revving the engine when warm to see if the coolant tank level drops significantly. It does drop about an inch when the engine is running and fully warm, but returns to normal a few minutes after switching off.

I'm hoping this is just a bad sensor, Does anybody know how to check these?