I had another, posh chassis candidate in mind, but whilst cleaning out the toolbox I found a chassis cut down from a Hornby 0-4-0 Thomas, by the looks and clearly destined for some failed project. I stuck it on the test track and it runs like a three legged dog, so it's gotta be a goer for the Neilson where such bad running is justified.
This being Purbeck, I'm reminded of The alternate world of Keith Roberts' excellent "Pavane". The cover art of my original copy sticks in my memory, with armoured knights attacking a traction engine and road train. I remember fondly that the engine is called the Lady Margaret. So for Old Man Lewin's Neilson clone, that's got to be a good codename. Hmm. That probably means that Old Man Wanostroch has a daughter or wife by that name. I wonder which?