Just an update/summary on this (solved) problem.
Initially my starter became temperamental; I could get it to work by rocking the bike back and forth in gear (which causes a minor spin of the starter, resetting the contacts for a next attempt)
When that 'solution' stopped working, I dissembled the starter and cleaned the commutator (the copper strips) using a meth soaked rag, reassembled it and everything worked as it should.
I noted then that one of the brushes was considerably more worn than the other, so I ordered a new set.
By the time the new brushes had arrived, a new set of problems was manifesting: again an intermittent failure to start, but this time coupled with strange electrical behaviour: a successful start often as not would cause the ancillary electrics (headlights, indicators, horn) to fail. I'd have to kill and restart until both worked, sometimes the 'rocking' method would affect the outcome, but basically it was a right ball-ache trying to get both engine and lights working. Ultimately it failed at night and I had to limp home with the hazards on (which still worked despite turn signals not working individually).
Armed with new brushes and a little reading about the Load Shed Relay (which apparently drains ancillary power during starting to make sufficient current available to turn the starter motor - hence the lights go out momentarily on operation of the starter button), I disassembled the starter again, ill-advisedly 'cleaned' the dirty-again commutator again this time using fine sandpaper, installed the new brushes, cleaned up the grounding (earth) bolts on the starter motor casing, and also sanded down the main battery earth connection when I was putting it all back together.
The outcome of this work was perfect operation of starter and all electrics since then (about 4000 miles).
Conclusion:
worn starter brushes cause not only starting problems, but less intuitively can confuse the Load Shed Relay causing diverse, intermittent electrical problems
Thanks everybody for the help and advice! It is very satisfying to learn things: as I've said, this is the first motorvehicle I've ever owned... Cheers! K