Well, I gave up waiting for a dry day and put my thermal pants on and hit the garage today.
Checked there was no more bits of carbon brush hiding in the points housing, checked the state of the contacts, reset the gap, then raised the throttle needle a notch, back to its previous setting. Now running like a swiss watch.
1/2 hour fettling, 1/2 hour riding, 2 hours cleaning all the crap off when I got back. Afternoon well spent.
Thanks to everyone for your help, you guys rock!