I checked under the bonnet today in day light. The fuse & relay box between the front wing and the air filter contains a 'deice' relay and a 'deice' 15 amp fuse. To gain access push the retaining clips inwards and lift off the cover....if you turn the coveryou will find a schematic for the fuses & relay.
Another thought is the condition of the contacts between the lifting rear window and the tailgate...I checked mine and if you lift the window and break the circuit the HRW light goes out....worth a check to make sure the contacts are clean...the cover on the window is spring loaded and should push back to reveal a series for copper connectors.
Thanks all, I'll check these but I think it may be part of a control module behind the Radio/central vents, I called in to a dealer who indicated that if a switch on the climate control panel is not lighting up, they all go through this rather expensive module [circa 750 + fitting and vat!!]