I note your advice to avoid flat batteries by switching off lights and removing the boot light bulb. It may be worth mentioning the bulb in the glove box, if there is one. In 2008, I had a flat battery twice before realising the cause. I have since removed the bulb permanently and have had no further trouble. The glove box is of course far more difficult to check than the boot light. You can always climb into a boot and ask someone to close the lid - but you can't do that with the glove box.