Thanks kommando, really helpfull !
1. Could it be the case that the bearing is too deep in the casing, so there is too much space for the circlip to hold down the seal?
2. How do you take out the bearing? Can it be done from the outside? If so, what do I need ?
1. The circlip locates the seal axially, ie in and out, it cannot hold the seal and stop it from spinning, this is performed by the housing giving a slight interference fit with the seal. The bearing may be located wrongly but that has no effect on the spinning seal.
2. You need to find out which of several gearboxes used on the B33 and get the service sheet for it to change the bearing (it goes by frame ie rigid, plunger or swingarm). The case will need heating and the bearing is driven back into the case, it cannot be removed from outside. But I doubt this is the issue as your gearbox would be locked solid if the bearing was too far in.
BSA did not print workshop manuals until the early to mid 60's, before then they issued service sheets.