Thanks again, chaps - query solved
The main problem was that the unit was bought from a peripatetic market man running a charity stall selling mail order rejects - hence no identifying details. Went to the same market again and he was back with another batch, so swapped knackered item for working variety.
'Once bitten, twice shy' rules meant that the first thing to do was to split the output leads into 'chocolate strip' terminals and meter the line voltages for future reference. Result? 12.1 to 12.7v (power off/power on) on one set and 11.7 to 11.87v on the other - so, 12v for cash.
I would imagine that this would be a standard for any similar units, should the need arise.
Now what shall I do for the winter?
