@Ilovecrumpets it has been a while getting it sorted - in our case the problems present more at school and the school were concerned enough to want extra funding for more resources but not concerned enough to actually do any paperwork
We started with a GP appointment where we outlined our concerns. I've found it helps to go armed with some factsheets on autism, ADHD, anxiety and any other related conditions you feel might be relevant and highlight where you see that your child's behaviours apply. I scribbled on notes as well - so where odd obsessions were mentioned I would explain that DS was obsessed with drains at the age of 2, lamppost numbers at age 3 or whatever you've noticed that stands out to you as being of concern. The GP listened and arranged a referral to the community paediatrics team. If he hadn't I would have sought a 2nd opinion. Another route for you might be to ask the school SENCO for help with a referral if your GP is unhelpful.
At the same time we asked the school if they had access to an educational psychologist or any other professional who could come and assess DS's behaviour in school. We had an initial meeting with the paediatrician from ComPaeds and after seeing our notes, the Ed Psych's report and talking to the school, she arranged a multi disciplinary assessment. This was a week during which we were at the hospital every day for appointments with the paediatrician, an occupational therapist, speech and language and audiology, a dentist, a psychologist, and finally a wrap up meeting with the paediatrician.
During the week we and the school were asked to complete paperwork as well which they used to 'score' DS in terms of things like his willingness to comply with requests, ability to hold conversations, empathy, all sorts of different things that on their own you might hardly notice, but all together can indicate different issues. At the last appointment with the paediatrician she gave us her conclusions verbally, the written report usually takes a few weeks to be prepared. We were also given a big stack of leaflets on local support services and other resources that might be useful to us but I imagine this varies hugely from area to area.
Next step for us is getting an EHCP in place so that we can hopefully make adjustments clear and required rather than the school being able to faff about half trying something and then not bothering any more without telling us anything has changed or push for a referral to one of the local SEN schools. Happy to PM if you want more info or just someone to chat to for support but I thought I'd put this here in case it's useful for anyone else.