Many thanks for all the useful advice. I found a procedure for timing from John Nash ages ago and followed that - with 5/16 BTDC, set at full advance, and a marked up straw in the bolt hole OPPOSITE the spark plug. My newly purchased TDC tool was hopeless - it's a side valve dummy! I had completely forgotten about that minor detail. I had marked up the points opening position on the contact breaker body previously though, and I could (just about) see the points open by eye.
Anyways - timing now seems better. Definitely now has an increase in revs at full advance after starting from a little retarded, and what seems to be a crisper exhaust note.
Not cleaned up the commutator just yet, nor ridden it, but battery now at 13V post charge.