I've got a 1991 XJ600 and on the way home today it broke down. First there was a loss of electrical power to the indicators, then a few miles later it lost power to the headlights and I heard a knocking noise. Stupidly, I continued on to see if I could nurse it home (a mile away), but the knocking got worse and the engine became jerky. I had to pull in and leave the bike because it became un-rideable. I tried to start the engine again to see if I could hear the rattle, but the battery was flat.

Does anyone know what this might be? Had the alternator chewed itself up?

just a guess really isnt it, i would be looking at the alternator too!