Well, I finally got to the bottom of it.
Double checked the relay and clutch, side stand switches, cleaned up the terminals just for good measure. No change.
So eventually this morning I got the starter motor off, disassembled it and a thousand bits of what used to be the terminal post insulator fell out the brush cover. Then i discovered why the terminal post seemed loose, the joining wire between the terminal post and brush plate is severed, looks like it's just been luck that enough of the wires have been inter-twined to make the thing work.
Anyway Cupar quoted me £52 for a replacement brush kit, so I reckon unless I can get something cheap on fleabay, I'll be ordering it next week.
No SV for me for the next wee
Cheers