I definitely didn't leave the lights on - that was the very first thing I checked and double-checked as soon as I found it wouldn't start. The breakdown guy checked for other obvious electrical drains (such as the boot light having jammed on, which apparently happens!) but couldn't see any, so he concluded that the battery itself had failed rather than merely being drained. His theory was that it had developed an internal short circuit, which he says can happen without warning if you're unlucky.