I read with interest all the possible things that can cause this issue on the various threads first and in the end checked the Radiator cap as a starting point.
It was old and when depressed stuck to one side at an angle,resulting I think in staying open and draining the water from the engine/Rad when hot.
Anyways up, a new cap (£2.50) and a good flushing of the system just for good measure seems to have cured the problem.
Time will tell I suppose, but heater on or heater off and idling forever on the drive, shes cool as mustard and dry as sticks now.
