Bob - phew! And thanks; again, I couldn't have done it without that tool.

Ralph - I the +1 thou was to allow me to get the seat in the hole; that was only in the test ali piece. I realise I forgot to mention in the main writeup that I re-cut the second diameter to the seat diameter exactly, so it fit nice & snug.
Eric - The shop I'm in now, at 4000sq/ft, has become too expensive, what with the rent, taxes, and electric. So, I'm downsizing into a 1250sq/ft unit instead. The racing cars and most of the other car stuff is going to another lockup in Wales. The combined rent + rates of the 2 new places is still nearly £4,000 per year cheaper than the rent alone at the place I'm leaving; so cost wise it's a much better prospect.
Fortunately, I've still got a decent sized space allocated to the machines, but I'm not sure yet how to lay them out.... that's the subject of another question, which I'm about to post on a separate thread...