I thought it was some sort of BSA sloper when I was trying to work out what the blazers type of bike it is. But it isn't.
The bike was made in Great Britain, Pommyland.
If Dad's old pal wasn't alive at the time I doubt I would have guessed it. But when he told me the make of the bike I had an inkling I had heard of them before, however never seen one in the flesh/steel.
Here is a pic of dads pal Wally on his bike.