"No diagnosis will help improve relationships / set behaviours in a family on its own"
See, I think this is possibly the fundamental reason why our CAMHS experiences went so wrong.
Our therapist couldn't grasp at all the fact that we have very good relationships with our children. We are very aware of their difficulties, their triggers, their strong points, and we work hard with them to help them reach their potential. We already went through years of strict parenting, which didn't work, so we found methods that worked.
All of our "care" was based around the fact that Dh and I didn't have a clue how to be parents. Because ds2 (and prob ds1) has PDA, the strategies that work are different, but in line with recommendations from the PDA society, the book The Explosive Child and similar to other parents going through the same thing, but because this didn't fit the standard mould of parenting, we were being wrong and obstructive, even though the parenting course was making things much, much worse (we stopped after we got to the point where Ds was attacking us, the course leaders offered no advice except to tell us to keep going).
Our reasons for going was worry about ds's suicidal threats, which happen when things get more tricky at school, where he masks like a pro. But in our therapist's eyes, masking doesn't exist, and because he was exploding at home, it means that the problem lies at home. We had no chance of being understood at all, because the facts were completely ignored.
CAMHS had their own agenda and twisted our words to meet their agenda, then presented that as fact.
We are lucky in a way because we know how to calm the situation, but taking Ds out of school every time it all gets too much is frowned on by school, but when they refuse to acknowledge his diagnosis and actually do something to support him, we have no choice.
We are also lucky that our wonderful GP understands the situation and has our backs. I've no idea where we'd be know without her.
CAMHS may be underfunded, but our problem wasn't so much accessing services, but dealing with their dishonesty and rigid unhelpful views, expecting all children to fit within their narrow criteria. The worry is though, if we ever need them again, if they continue in the same vein it may damage our family, and they are already calling us obstructive, for doing what we know is the best for our DC, because they want us to parent by the book, they want Ds to neatly and obviously fit in the manual (which he doesn't).