Wow! (<- insert popcorn munching emoji here)
Thanks. Dave. I'm playing around with this when not playing with the steam engines you sent me.
The Mamod loco was meant to be spares for a disassembled one I already have, but yours is better than mine, so it'll work the other way around, I think.
Did you notice that the MM1 seems to have a non-standard piston? I have another of these, and the piston is considerably different. It doesn't really matter, though, because both of them just scream along.