Have made progress and the 'clutch chainwheel + backplate' are now off. The chainwheel plus the double row of ball bearings did not come off easily and needed some persuasion (forgive me, I'm not an engineer and doing this job with a spanner in one hand and the Haynes manual in t'other).
The 'immovable' clutch centre' (or clutch sleeve collar as it's called in my BSA Spares list) is on the other way round, ie wings inwards, to that shown by Haynes (and the spares list for that matter).... as it is keyed AND sits on a taper I assume the illustrations are wrong ???
Shall need to obtain a puller to aid removal.
I had assumed from the exploded diagrams thar the 'clutch push rod' simply pulls out (from the primary chaincase side) but of course mine slides out approx 1/2 inch and then won't come any further (locks solid) ...... why does everything 'resist' ? Do I pull harder or might there be a burr on the gearbox end which needs 'dressing' ?
Thanks in anticipation
Peter M