What you say makes sense, thanks. I turned the lights on when the bike was in the shed last week and thought the beam was a bit feeble, and slow to fully light up. I put the Optimate charger on (after disconnecting the earth lead), but it went straight t green, ie: fully charged. Are these charger lights reliable? The battery (a Motobatt) is 8 years old, so possibly getting towards the end of its life? Is there some way I can tell for sure?
That often happens when a cell or two is dead. I have several battery chargers that have green lights for fully charged batteries and they all do this. You should do a load test on it to see what the voltage is after a minute or so of full lights on. If it doesn't bounce back to over 12.5 volts, the battery is probably near the end.
I have a ten year old battery in my old Toyota. It never shows more than 11.8 volts after this test. I put the battery charger on it and the light goes green in just minutes. It's going to strand me one of these days, but it keeps starting the car and I'm cheap so I'm tempting fate.