[rovertone]

I have been having similar problems especially with my wife's 25, I have an easy 10 mile 'round the block' trip which helps once every 10 days or so.

Interestingly I had to buy a new leisure battery for my caravan and the supplier, a specialist battery stockist specifically state that they do not honour stated warranty on car batteries where the vehicle averages less than 30 miles a week!!
In the same vein I called an independent auto electrician about the 25 who said he wouldn't waste my money by coming out to the car, he spent many years working for a large auto electrical company before they closed up and he said Rover 25's & 45's were 'regulars' for parasite drain checks and in practically all cases it was due to the cars lack of use.
I guess we all take our batteries for granted but current times are making us take a closer look and as Simon says the current low temperatures are not helping.