Well I'm no longer confident about the diagnosis. When I measured the voltage on the battery at lunchtime today it was down at 11.94 but having taken the bike out for a spin, it's up to 12.3. Someone correct me if I'm being an idiot, but that implies to me that it's charging OK! I have no clue about electrics, but I guess this means that my battery is draining whilst the bike is not running. I'll check the voltage again tomorrow evening, but beyond that, what should I check next?