With the summer fast approaching, my step dad has only just got round to getting someone to look into the problem. The replacement multitimer did not make a difference so we conceded defeat at this point.
The car was booked into a local VX dealer (Purseys nr Street, one of the better ones from my experience). Anyway, they could not trace the fault so re-wired the feed to the circuit from another, suitable switched 12V supply. This restored operation. They have suggested some corroded wiring, due to the age and mileage of the car (10 years and nearly 200K). I have a feeling that one of the connectors in the engine bay may be either badly corroded, or at fault.
Anyway, problem solved, albeit in a slightly bodged (yet safe) fashion.