Well, I managed to get a couple of afternoons in the workshop this last week. (Do I hear a chorus of "about b****y time?)
I spent two afternoons checking the axle centres, and found that the one Steph had marked up was one of the front hornblocks, not the rear, which is where all the problems appeared to have been. I made lots of measurements, checked stuff on jigs and actually spent a whole afternoon with the long ground steel rods trying to get a handle on where the problem lay. I should have just accepted that the man-boy guru got it right to start with (I still don't know how!), and the dimension which was awry was one of the front hornblocks.
The soldered joints around the hornblock were protected with water soaked bog paper (if you've not tried it the bog paper can hold a lot of water and can be moulded around any areas you want to protect) and the gas torch was applied. Having cleaned up the chassis and the hornblock both were reintroduced to one another on the jig and re-soldered. The result was exactly as before, as it was on the next try, so for some reason this hornblock is "floating" itself in to a position other than that where I set it up. Anyway, on the third attempt, and by applying a bit of bias during the soldering everything measured up quite reasonable.
So I reassembled the coupling rods and motor, and I now have a running chassis!
'Nuff said, and thank you Steph.
(Well, perhaps I should say that the problem has nothing to do with the kit and is all down to my reliance on a chassis jig......)
B