Hi Johnu
A not unheard of problem !! You do not say which type of clutch is fitted 4 or 6 spring ?
Adjust the mechanism and cable according to the service sheets,https://cybermotorcycle.com/archives/bsa-super-rocket/service.htm
The gearbox top lever should operate through 90 degrees to the cable as should the "pusher" inside the gearbox outer cove inspection cover. Is the handlebar lever standard and the pivot unworn?
General wear of the clutch components can cause the plates to stick, (ridges on the drum and hub and worn plate tabs)
Is the oil level correct and is the oil of a suitable grade?
If the bike has a 6 spring clutch Magnetoman has a very detailed rebuild thread here,
https://cybermotorcycle.com/archives/bsa-super-rocket/service.htmJohn