So, it's all back together. The drive shaft now 'telescopes' like it should (it was solid!), the needle bearings in the right side of the bevel box actually move (they too were utterly corroded to a single mass), and everything else has had a good clean and grease. The old drive shaft bolts were munted and I think that's because the shaft wasn't moving. Could be my imagination but gear changes seemed smoother. More snick-snick rather than clunk-clunk.
Thanks for your help along the way with this. Much appreciated, gents.