Gentlemen, thank you All.
They say a picture etc, so here is a rough sketch:
The main thing is that the hollow tunnel in the camshaft has two diameters.
When I fitted the bushes - labyrinth seals - I drove them flush with the end of the camshaft, as at the timing end in the sketch. Paul, my worry was that they would be
so tight, they would only go part-way in and get stuck. But it was OK and they went where I wanted them. I assumed that, as the camshaft cooled down and the bushes warmed up, the bushes would be an interference fit and stay put.
But yesterday, as we would on the nut on the Boyer timing rod, the bush at the advance end was pulled inwards. I don't actually know if it has moved all the way, ie up against the shoulder where the tunnel narrows. Will try to have a wee feel with a wire hook or similar.
Obviously, when we stopped yesterday, the Boyer stepped spacer was up against the bush, then two washers, then the 13mm nut which had almost disappeared into the tunnel.
One solution would be to drive the bush back out. But I think that would be difficult and risks damage, at least to the bush. And cannot guarantee it wouldn't move again.
Or, could buy another labyrinth seal, or even two, and fill the wide parts of the tunnel. A good solution, but expensive.
If I had a lathe, I would turn up a simple bush. In fact, that's what I would have done to start with.
But today, I will have a look and see if there is anything in the garage that might fit over the Boyer 8mm threaded rod to take up some space. Stepped spacer, then this 'sleeve', then the washers and finally tighten the nut. That should be OK.
As long as I don't over-tighten and move the other bush too . . .