I've just this morning got this message back from a retired workmate:-
You have probably solved this by now but in case not... We think you are right. it's a BSA C11 between 1946 and 1952. Lightening up the image shows a chrome strip on the tank and what appears to be an oil filter in front of the base of the cylinder. Compare with this - http://www.classicmotorcycles.org.uk...50cc_rigid.jpg This looks likely; rigid, chrome strip on the tank single seat.
After 1952 it had plunger rear suspension.
Cheers,
Looks like the hawkeyes were right
