Hah, looking at that epinion now. Although he never explicitly says so, the gear must have broken on him too: "...and I haven't had any issues to speak of until last night.", and then later "I learned from this. I have 4 extra worm gears on hand ".
Thanks for the suggestion on casting. That's something I've been thinking of trying my had at, and maybe this will provide the right motivation get on with it.
FWIW there's a pic of the genuine replacement gear here:http://www.ereplacementparts.com/worm-p-189810.htmlhttp://www.ereplacementparts.c...
although I note that the advice offered in the comments is dead backwards, 726713 is definitely the raise/lower gear.
Thanks again for the comments & advice.