Thanks guys though what I actually did was...
Don't read on if you're a little squeamish or a rivet counter...
Drilled two small holes in the frame (one for top and one for bottom race) that allowed me to use a punch to push the races out. To explain more fully. for the bottom race, I drilled a hole in a similar manner as the grease nipple but so that it came through at the bearing seat instead of above it.
Lost a couple of drill bits in the process as they obviously went blunt when they reached the bearing.
I'll now tap a thread and insert a grub screw so that, in future, it can be done again
I did consider the structural integrity of the whole thing but very much doubt there will be any problems.
Pity BSA didn't do this in the first place