Right, I think I might have sussed it, but would like confirmation from someone who knows.
This was the problem - I couldn't get the half shaft in that last 1/8".
I bought the axle off flea-bay and it came with the hubs and half-shafts separate from the axle casing.
Working from the exploded diagram in Rave and the parts manual. I ordered all new bearings, seals, washers and nuts. The parts manual listed the spacer washer, which goes between the inner nut and the outer bearing / seal as "FTC5241 Washer Stub Axle" which is this -
...it's about 3mm thick and is the same O/D as the bearing seal.
However, my axle came with a different washer, loosely fitted between the 2 nuts on the stub axle. This is about 6mm thick, had an out-moulded rubber seal on one side, and fits inside the outer bearing seal.
... in this picture you can see it behind the nut.
This, thicker, washer doesn't appear on the exploded diagram in Rave or in the parts manual.
If I fit the thick washer and the thin washer under the inner nut, I can't get the half-shaft all the way in. If I omit the thin washer I can.
(The O/D if the thin washer is bigger than the I/D of the drive member.)
So, like this, the half-shaft won't fit...
Like this, it will...
Have I got it right now, even though it appears different to Rave?
For Buster's reference, this is a Rover rear axle half-shaft - it's all one piece with the drive-member.