Good morning,
I can see the logic in asking alternative frame builders for an opinion, but I can't really see what is in it for any frame builder to answer.
In the UK there are relatively few frame builders, so the idea of getting involved in criticizing another builder and then possibly having to back up the opinion in a small claims court is a no brainer; decline to provide an opinion.
Unfortunately I suspect that you are going to have to go either the CAB or a solicitor for legal advice as this is a bespoke order and the builder may quite legitimately believe that is has been fulfilled and you have no right to cancel it.
It would be nice to see a photo of the stay as my understanding of the quality issue if you go legal is that it based on "what a reasonable person would expect" not on any absolute standard.
Bye
Ian