Very nice picture! If the serial number is what I think it is C4353082. Then its one of 17,000 bikes built under contract C7287 which were demanded in May 1940 and completed in Aug 1941 (So a 1941 bike as John Suggests)
No offence, but I would hazard that Granddad looks a bit old and out of context and maybe just posed on someone else's bike. Do you know his age and history Maurice? Ron