It runs!
I spent most of yesterday afternoon fettling the motion. The challenge was to get the thing running as smoothly as possible under power. Of course, that meant the coupling rods and inside motion had to be fitted, tested, taken out, fettled, refitted, tested, etc. I reckon it was in bits seven or eight times, with some more this morning.
Of course, being a fool. I'd painted the inside motion before testing it properly. Following Steph's advice to "slosh some Brasso over it" meant the paint came off again! But it did the trick. Tight spots on the eccentrics have gone, and likewise for the crossheads - though some unorthodox surgery has been required to ensure the latter don't end up binding on the front axlebox bearings.
Eventually, I could push the whole caboodle along the bench with everything wiggling about in a fairly satisfactory manner. Now, I'm letting things bed in on the rolling road before I take it all out again to reapply some paint!
I have to say, even though this setup is essentially idiot proof, I haven't enjoyed the process. My usual preference for commissioning an expert to do this kind of work seems vindicated. It's been an interesting and frustrating exercise, but I don't think I'll willingly do it again.
Right, test match from Lords on the wireless, Boycott is wittering on, and it's nearly time for lunch!