Originally Posted by Steptoe
The starting failure sounds more like a starter motor problem.
There were two separate problems:
1. The D+ spade terminal at the alternator was loose. Tightened up and reinstalled and the alternator starts first time, every time.
2. The starter motor was binding sometimes, drawing lots of current and causing the battery voltage to drop during cranking. This also means the injectors don't fire enough fuel, causing it to crank longer before starting.
Took the starter out and broke it down, in pieces. The model is D6RA75 which I believe is the "good" one.
What I've found is that the inside cover to the planetary gear system had somehow fallen out from it's location covering the gears. It then ended up attracted to the permanent magnets and found its way to the top of the armature. The piece I'm referring to is the cover you can see in the (copied photo from another post) picture below. It has been rotating with the armature, part of the inner hole has worn off and there are signs of arcing (including some holes in the cover!) but otherwise it looks fine. The armature wiring that made contact with the cover had some wear but is okay.
I reinstalled the planetary gear cover as in the photo below, put the starter back together after a bit of lubrication and cleaning. The motor now spins faster, is quieter and the battery voltage stays higher during cranking. The bike starts more quickly too.
Thanks everyone for the help.