I spoke a little prematurely as some further test running showed a tendency for the rear bogie wheel to short against the head of the M1 bolt, which I think is fractionally thicker than the supplied one. However, some very slight adjustment of the cylinders and slidebars cured that. The philosophy is to keep testing each element before proceeding with the next. I also soldered up the two etches on the crosshead, so that they don't flop around at odd angles to the slidebars, as in the above image.

I'm now trying to work out the best sequence for the radius rod and all the return crank gubbins. I've also decided that the motion bracket needs to be tucked back under the footplate valence a bit more, if at all possible.