Looks like its going to be something along the lines of a Francis Barnett trials or competition bike ?
This is not exactly the same, but something close. ?
This is said to be circa 1950, but its more likely later. Yours too, but not as later ?
http://cybermotorcycle.com/gallery/francis-barnett/images/Francis-Barnett_1950c_NV2.jpgVery popular in their day, for budding compy riders.
And much more manageable than the big bangers !?
Oddly, it was that fuel tank cap that instantly brought FB to mind.
A lot of bikes back then shared a lot of parts, you may find many common to other makes.
The engine and gearbox in particular, and probably forks and wheels and brakes too.