As I suspected, there was a "distance piece" behind the clutch sprocket. I have chucked that in the bin and now the clutch barrel locates as it should and Its all gone back together very nicely thanks.... however I'm not absolutely sure that I have enough end-float between the inner and outer clutch baskets even without any shims fitted. There is a discernible gap that I would guess to be about 10-15 thou (can't get the feeler gauges in!) so I'm hoping that will do. How would one obtain more end-float? The securing nut is done up firmly, but not tightly - is it ok to leave this loosish but locked with the spring?
Next step is to make a new clutch push-rod - I've bought some 1/4 inch EN8 mild steel rod as I'm told this is best for home-hardening. Would people recommend a one piece or two piece push-rod? and, if two, should I put a ball-bearing (1/4 or 5/16th - both seem to fit in the tunnel?) or a 1/4 x 1/4 roller bearing between them? Many thanks for your patience - I'll get the hang of this eventually!