Sorry it's taken me so long to get back to this thread.

The problem was that the voltmeter picked up it's supply from the rear wiper motor switch on the dashboard. One of the sparkies at work had meggad 'a wire that was live with ignition' for me to supply the voltmeter with - rather than run it from the battery and it being live continuously.

Is there a better source for the supply to the voltmeter? Please advise. Thanks