I souped up the IPSEA letter. The letter was about 6 pages and the bundle ended up nearly 50 pages with all the reports and notes.
I'd been told by everyone that we wouldn't get anywhere but I made the best case I could and the LA agreed to assess. Now waiting for the draft plan and looking at getting extra funds for school to support.
It's worth putting together something comprehensive IMO if your child is not failing academically and has more subtle/obscure SEN.
I added in a list of the various professionals relevant to DS - mostly NHS. I added a list of referrals made but awaiting first appointment. I added another list of people we might be seeking advice from.
I did a list of the "4 areas of need" and I put down a brief list of DS's difficulties under each of the headings.
I then took our main concerns (4 from memory) each in turn and presented evidence of "un-met" needs. Also the need to avoid future problems as ASD kids are high risk for mental health problems, so I highlighted the early warning signs that he was showing which I thought showed he needed support.
I included letters from professionals where I had asked them if they would write to support an EHC assessment and/or plan for DS. I used quotes from school staff and specialists highlighting where he would not meet academic potential. I used my observations, teachers' notes and SALT comments to highlight where his behaviour was showing anxiety - non compliance/destroying clothes/fidgeting etc etc I included his diagnosis report and a feeding disorder service letter. The school SALT had done observations and I had noted her comments about his poor pragmatics and that it would cost him socially in the future. I painted a pretty bleak picture, which wasn't an easy process.
We had a sensory OT report done privately thinking about doing sensory integration therapy. We decided to go DIY in the end but the report was useful as it showed he was delayed in fine & gross motor skills and social/emotional sphere. We couldn't show academic delay so I felt I had to really get lots of other evidence.
I put in a couple of study references on poor outcomes for non compliant ASD kids at secondary transfer and that an ASD friendly school would be essential - ie choice of school based on need rather than distance.
At the end I highlighted the 2 part test for EHC assessment which IPSEA list out on their website - with the formal reference - a child "may" have SEN and "may" benefit from a Plan. I said the ASD diagnosis showed unequivocally that he had SEN. I had a letter from the NHS psych supporting possible benefit of a Plan. So I finished up by saying that we clearly met the 2 part legal test and I expected a reply from them in 6 weeks (gave the date). I wrote to the head of children and families at the LA and cc'd the SEN team.
Good luck - the IPSEA foundation law course really helped me understand what to write and how to use all the jargon