Wow, I will just start by agreeing with Pete and say what a fantastic and helpful set of responses to reply to individually, so a collective big thankyou!
I realise on looking at the packets the gears came from, that only one of the gear set I am thinking of utilising is in fact Association - the others are from different sources eg Mikroantriebe of Germany, and I think, Nigel Lawton?
The small 12T gear was originally designed for a 1mm axle which I have opened out to 1.5 to match the 28T - which I am sure is the association one? All 100/ Mo.25's by the way.
The worm appears to mesh fine with the 28T though it is not a helical one. Will that be a problem?
Also - the 37T is too big as suggested, so I have selected a 30T instead, though I do wonder if that is still a tad big, at 8mm diameter, for these 8.5mm wheels? I have wedged the wheel slightly into the ###### in this photo. I notice that the 30T under magnification looks a little worn anyway, so will need to replace it...
Thanks once again - oh and Missy, you explained that very well!