A tiny bit more progress.
The worm shaft needed top hat bearings, and at the third attempt, I won some on eBay... nobody seemed to want two packs in one auction, so, I've ended up with 44 (One pack had 24 in).
And to compound the 'Metrinch' saga.... they're 2mm bore, 1/8" OD!
Anyway... after a bit of sawing, filing, hammering and swearing (not necessarily in that order), I have a worm shaft with 2 bearings trapped between the worms, which I'm not 100% sure I'm going to use now.
As the sides were ¼" + 4mm + 4mm, with the axles on the lower 4mm centreline (ie. 6mm down from the bottom of the side channels) and the distance between the axles and worm shaft worked out to be ¼", I realised that the worm shaft had to be 6mm below the top of the chassis. With this much required new info., I cut two ½" pieces from the rectangle I'd used to space the sides apart, and drilled an 1/8" hole, 6mm from the top edge. I opened one out into a 'keyhole' slot to aid assembly/dissembly (see picture)
With the axles in, wormshaft in, bearings in place, I made sure everything was 'free' and trickled liquid poly around the two new bulkheads. After the glue had dried I popped out the axles, slid the bearing out of the keyhole, and the worm shaft comes out with a bit of wiggling.
Stop Press: I have (after waiting 4 weeks and 3 days) received 24 figures from China. The driver will be 'The man with a suitcase' as seen here: http://forum.gn15.info/viewtopic.php?t=238&start=300 about halfway down page 13. In unpainted guise, he is in 4 pieces. In the pic his head is held on with a tiny blob of BluTac. The case is only part of his right arm, and looks easy to remove with a bit of 'sawing/filing/swearing'. He's hard plastic, and I've already used Humbrol Liquid Poly, and Squadron Green Putty on him/two of his friends, and both stick perfectly. I suppose I'll have to start another thread about him. Oh, and he is sitting on 3 offcuts of ¼" square, which determines the seat height as ¾" above the footplate.