Well actually, it's a 1937 B25, but the 'box (or at least the selector mechanism parts) are the same as the M20 box.
The gearlever stuck solid while I was doing a test ride around the garden. Careful disassembly showed the the selector pawls were BOTH in engagement ie the necessary "free play" to allow the mechanism to shift up or down was absent. Inspection of the various parts showed that the pawls appeared unworn, as are the matching notches in the ratchet lever. All parts appear present (by the original and year-correct BSA parts book) so is this a known problem?
Thanks for any help.

email (option): kevin.sandford@ntlworld.com

Hi Kev..Have you checked the gearchange return springs on the back of the footchange mechanism?...Ian

email (option): ian@wright52.plus.com

Hi Ian, yes, both present and correct.

email (option): kevin.sandford@ntlworld.com

Weak springs & sometimes that outer can be dry on the inner shaft after a long period of non use.
Ive copper greased that bushing in the past on boxes to locate the cause & found the springs cope afterwards.