Hi Scotty,
Thanks for all your help, I couldnt have got this far without it.
I've worked out the problem (i hope)
The brake drum I have is an early A7 plunger or rigid, it uses a much larger bearing, presumably as it is subject to greater loads.The extra thickness of this bearing effectively shortens the reach of the stub axle. Whilst I could probably get a mate to make up new bits and spacers in exchange for a decent bottle of malt, if it still doesn't work, I wont know if its my calculations that went awry, or the theory that was wrong. I'm going to bite the bullet and buy a new drum.
Cheers,
Iain