Folks, a small update.
Got the munted worm wheel off the apron assembly, for a little dressing and dimensional characterisation.
Worm gear looks to be 3 degree helix angle, and so with the help of a colleague got a solidworks model of the worm wheel done up and 3D printed.
The point of this is to ensure I don't spend proper money on a new gear that is not quite the right spec.
The 3D printed unit fit well, and worked under cordless drill power.
Gives me sufficient confidence to go get a proper one or two made.
Will do a bit more testing to see what the wear patterns on the plastic gear can tell me about any wrongness, but at this stage I'm ready to get one made that's not plastic.
Cheers.