Actually Nige I think it was reading a post from you a while back about not being able to dismantle the motor that made me go and buy a new part before I tried fixing the old one... you owe me £200!
Cheques in the post...... but only for £50 'cos that's what I paid for mine !
Your experience does sounds familiar, I seem to recall it was something like that, perhaps with more patience I may have done it but like with many things these days I can't be bothered ! I don't think that many people would have managed a rebuilt like you did though so top marks there.
I don't think a repair would be possible though if, like mine, the magnets had come adrift, would have been a bit of a bodge which may have lasted years or maybe days.
Now not to worry the OP but when mine started to make " clunking " noises it failed after about five more operations, halfway down, or up, depending on how you look at it. Straight home to find blocks of wood to hold it up, after the fun of finding all the screws to remove the door card of course. Wished I'd known about this site then, would have saved myself a lot of grief !
Seriously though I reckon that there would be a ready market for any spare motors you have. Wonder if you could source new motors from anywhere, they'd certainly be a popular item as window problems certainly aren't unusual.