[ajgriff]

Plucked up courage to peel off the self-adhesive polarising filter only to discover that the panel itself is the cause of the blotch. Still no evidence to suggest how the damage happened in the first place. On a positive note I've managed not to cause further harm although I will need to replace the filter (film now on order).

For fun I'm attaching a couple photos showing the main reason for the meter's non-working status when I bought it. Actually think that the battery (zinc-carbon?) is the original, dating from circa 1986. The second shot shows the PCB after spending an hour or so carefully scraping away the crud left behind by the corroded battery. I've since improved things futher.

Alan