You will no doubt be as relieved as I was that the car started OK yesterday, and I was able to get to Kwikfit for a battery test. Interestingly, the machine showed nothing wrong with the battery or the alternator, so it must just have been the fact we don't use the car in London and it's been cold. Sometimes this can make a battery discharge. Because the battery is nowhere near dead yet but needed charging, I did a bit of a drive around and went to Comet (the electrical people) and bought a 500GB hard drive to store our photos.
So now I have somewhere to keep my photos, and (hopefully) a car that's all topped up with electricity.