Optimistically I start the motor with all 6 axles in place. Not a flicker. Turn the power up a little... still nothing.
At least when I add the 3 wheel muffs (still on makeshift axles) it all runs reasonably well, but still not smoothly.
Eventually decide to check the worm wheel axle. On close inspection there is some friction between the muff and axle bearing. I take it out, file the end of the muff and
glass-fibre brush the axle bearings. Do this a few times, with improvement each time. Perhaps that was the problem all along.
Some lubricating oil on the worm wheel also helped. As did unscrewing the motor a quarter-turn from its bearing, but now the motor had to be aligned accurately. I would need to take care when securing the motor to the frame spacer underneath.
Using strips of card and paper, I find that the worm wheel turns fastest with 0.4mm support. Stick this onto the spacers with double-sided tape as a temporary measure.
I don't want to add the wheels and coupling rods until I've got the gears moving as well as possible, so meanwhile I'm going to start on the underframe.