We reached the point you are at. Our Clinical Psych suggested PDA, but was unable to persuade the AHA to fund assessment at Elizabeth Newson Centre.
We provided school with the overview of PDA (fits DS1 like a glove), and the educational guidelines.
School have taken them on board to the letter. In y opinion, the provision is of far more importance than the diagnosis, as if the provision suits the child's needs, it makes no odds what the dx is, but a dx without adequate, needs-led provision is useless.
Another thing to bear in mind is that, even if you pay for an assessment, and they say he fits the PDA profile, this is still not classedas a diagnosis. SInce PDA is not recognised on dsm5, and is not going to be recognised in dsm6, PDA is not deemed to exist 'officially', and so cannot be 'diagnosed'.
There is alot of research going on at the moment, though, and this can be useful in understanding your son. We took part in a research project and the student (doing her doctorate, so knew her stuff) came and gave a really thorough set of assessments, which she followed up with a very in-depth report. The report was useful to read, and I would have jumped on it a year ago, but becaues DS1's behaviour has calmed down so much since going to this (wonderful, amazing) school, I simply filed it, as it doesn't add anything we need at the moment. I can't believe we've reached that point! 
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By the way, Ds1 is doing so well because he has a full time statement and a completely individualised timetable. He is physically in the mainstream classroom, but has his own timetable (very flexible and all negotiated), and fantabulous support, which the school has trawled the entire area to find.
At his annual review, I asked if it was likely they would reduce his support hours and the SENCO almost laughed at me-she said no way could they have achieved the success they have done without full time 1-1. They even wrote into his statement a list of 'ideal characteristics' for anyone who is employed as his 1-1!