Last weekend I checked everything, with my brother-in-law's help, and we found that the HT coil was cracked. This seemed to explain it, so I ordered one and it came during the week, and I put it in on Sunday. Charged up the battery, bike started OK, problem solved.
Except that tonight when I wanted to go home the bike wouldn't start. Period. Tried everything I could think of, and it was acting like it was short circuited somehow. Ended up calling the AA. Explained my theory to the guy, who got out his jump leads and gave it a ten minute charge.
Upon which it started perfectly. Once. But as soon as he disconnected the jump leads it became obvious that my battery simply wasn't holding its charge. Everything else is working correctly, and the bike is attempting to charge it, but Mr. Battery has shuffled off its mortal coil. It is a dead battery. After about a year, and a cost of 60-odd pounds (bike parts are always expensive). This is what I get for being a low-mileage rider in a cold country, of course.
So... working tomorrow and wednesday, so I won't get this fixed until Thursday at the earliest. Which makes it highly unlikely that I'll make the pre-Christmas Tun, apologies to anyone who was expecting to see me there.
On a related note - for some reason I already have two old bike batteries knocking around my flat, the dead one will make a third. Any suggestions on who to contact re recycling them?