Acton Scott Victorian farm in Shropshire have a programme of courses including working with heavy horses as well as one with a donkey. They also have an excellent harness maker, Terry Davis, come and do a harness making course which I attended last year, we didn't get as far as collar making, just made a headcollar. Terry is based in Shropshire and would be worth speaking too, and could obviously make a collar to fit if you wished to purchase new, he has so much experience.