Gromit,
The symptoms you describe is exactly what happened to my Electra.
Bike was started and was ticking over (very low). The theory is that when it stalled, it actually tried to spin backwards.
Following that, the starter just whirred, obviously connected to nothing. It turned out that the Sprag Clutch had siezed and would not engage. It had to be replaced.
The Sprag Clutch is a "one way" device that allows the starter to spin the engine but won't let the engine spin the starter.
Replacement is not too comlplicated, but you need to have an idea of what you're doing and a couple of specialised tools.
My general advise is to avoid the low tickover, especially when the engine is cold as it is more likely to stall and repeat its self destruct programme.