ok. developments...
Found a loose bolt on my crankcase last night, though can't remember which bolt it was. May have been the refill bolt, may have just been a bolt to hold it together.
Either way I replaced my transmission oil last night. This morning on the bike, I had 5 miles low power first, then 3 miles good power (if a little poor at low speed), all on the same journey.
So, may have been transmission oil, but still suspect my battery - my lights dim and flicker pretty seriously when the bike is idling. If not the battery, and not the coil (recently replaced), I guess this may be some other electrical issue. Will get the manual out.