It's been virtually a month since I posted on this, but things have actually progressed somewhat in that time and it does at least run now! I'm still not entirely happy, but hopeful that the last little hesitation will disappear with some real running in.
So this is where I am at, today. The roof is still loose until I've installed the cab fittings, and of course it still needs the springs, various bits of pipework, cab steps and buffers/couplings, etc. Not to mention a driver.

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I hope this shot might be of some use to others.
Right back near the beginning I mentioned that I had a plan for the wheels. One of the things that always slightly bugs me with 3D printed wheels is that they are always all black (or white, brown or grey), whereas wheels that are in service always have shiny treads and at least some shine on the flanges. You can of course paint them, but the paint soon wears and also seems to attract all the track muck. Sooo, I bought some Sunlu "Silk PLA+" in silver and used that to print the wheels. It isn't perfect but to my mind looks a lot closer to the real thing.
As an aside, I used the wheel drawing to create a mask for painting the centres and backs, which worked really well. All I did was to put a 6mm thick 3D box around the wheel drawing and then delete the wheel, leaving a correct size and shape hole in the slab. This fits over the wheel and allows spraying from both sides without getting wobbly edges.

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Also clear in this shot are the slide bars. Brass ones don't look right, to me, and I'm not convinced that printed ones would be robust enough, but after a few mins on fleabay I came across some 2mm square keysteel and that fits the bill perfectly.
Finally, the piston rods are the pins from suitable sized pop rivets, with a brass sleeve inside the cylinders.