OK, another afternoon dedicated to this hopeless task… but I’m going to report what happened here in case it helps Rob or somebody else who’s trying to do it.
The hole for the gear to locate in the case/cage which holds the motor is worn oval, so I assumed that was the root of my problems, and decided to move the motor and its little PCB to the new cage I bought. I’ll try and post a pic, but the site will only host a handful, so it depends on whether there’s something I can delete or not.
The little crimped bits on the ends of the two wires which connect to the motor are actually tiny spade terminals – you just pull them out
of course I discovered this after I’d cut them, then realised that they still wouldn't clear the cage that the motor sits in, so I just gave them a pull to see what would happen
Anyway. So I fitted the motor to the new cage, soldered the wires back on etc – it spins very happily when connected to 12V. Greased the new gears, and located them carefully. Put the lid back on, giving it a bit of a wiggle to locate the gears OK.
Applied 12V… a clunk and a graunch. This began a saga of taking it apart, moving things slightly, putting it back together again.. and again… and again. No joy; sometimes the motor spun a little in one direction, but not the other; sometimes just grunted and clunked. Of course you're working blind, no idea what’s not meshing properly inside, so all you can do is keep trying. I even tried the old gears in the new casing.
After maybe 20 goes, I admit defeat. I mean, there’s no rocket science in there – just 3 little gears which turn each other.
I give up…