I really don't want to be rude but had the post read "Have you tried asking the GTA?" that would have been a nice post and clearly someone trying to help. 8x57's post didn't and still doesn't read like that to me. Perhaps I am misinterpreting it.
The answer is yes, the gunsmith in question has tried the GTA. He has been given a few names that can insure him. I wondered if any gunsmiths out there had had any particularly good or bad experiences with insurers as there seems to be a huge range in both quotes and the amount of paperwork involved, as well as the level of cover. I should have been clearer in my original post.