Update on battery drain culprit
I opted in the end for a new Bosch S5 battery on the grounds that it's not far off the cost of a full tank of petrol these days, and then had a few modifications done.
1) There's now a battery isolater key switch in the small cubby hole in the boot, so if I ever need to leave it for a few weeks, I can turn everything off.
2) The centre console now has a new solar panel charging socket so I can plug the panel in over the winter when I don't drive her much.
Alfamania tracked down the most likely cause of the problem - the socket assembly feeding the electric mirror adjustment panel had clearly burnt out at some point in the past and this was something I had mentioned as a 'todo sometime in the future'. On investigation though it seems that the slightest knock was causing the short to put the instrument panel lights on and so it was most likely leaking a little all the time, even when there was no obvious sign of a problem.
So now I not only have a working trickle charging option, I've got a pair of adjustable wing mirrors into the bargain!
I guess the point of my post is ... if the battery isn't holding charge, there is likely to be a good reason!