Thank you so much for the kind and detailed responses. I really appreciate the perspective from other mother's who can relate, and people with ASD themselves. I've taken in everything you ladies have said.
One of my biggest fears was highlighted on the thread and that's the potential for us to be reported to social services because of the head banging and wailing tantrums that last over an hour. Fortunately, the health visitor has had concerns about ASD since he was 12 months old so I don't think she would ever think anything sinister as she knows him and his temperament.
We live in S.E London so any pointers for resources would be fab, preferably resources who don't mind where your GP is located. I don't fully understand why that's an issue but it's something we keep coming up against. I don't want to change GP's as it's our family doctors, they're brilliant and know us all well.
A quick Google search came up with a great service but the sticking point being they only take on and support after the diagnosis has been made officially which is disappointing.
We were under kalidoscope in S.E.London for all of one appointment then discharged when we told them our GP was in another borough, so that voided us for the support we could have got via them for example SALT, paediatric referral etc. They were recommended by the HV but she wasn't aware the GPs borough would be a problem.
I'm going to look into weighted blankets and protective helmets for when he's having a meltdown. Usually I will try applying some pressure to his body by holding him tightly and soothing him but it rarely calms him down and I end up being headbutted 
It's very difficult to get out and do nice things as a family because once he's out the pram he just wants to run and run, this is an issue for two reasons the first being safety and the second is because he has tibial torsion and another condition which means his feet are bowed so he always falls over.
Any attempt to encourage him to focus or take part in any activity were doing results in him shrieking and kicking off until we're forced to abort the day out.
We attempted the zoo and sealife the other weekend and it was disastrous.
I love him beyond words but it's so so difficult sometimes. I feel sad for him because of all the opportunities he misses out on because his condition makes it impossible to do and enjoy things like neurotypical children.
I wasn't aware we could claim DLA for him prior to diagnosis so thank you for clarifying that we can.
It's quite isolating at times because I can't take him to the park and socialise with other mums in the way that mother's of NT children can. I find that when I do take him to the park he just wants to stand and swing the gate for an hour and avoids the other children like the plague. If I try to encourage him to join in with the other children or do something different he'll have a huge meltdown and this is often met by a disapproving look from other mums there.
I would absolutely love to find a group solely for ASD children where he can be himself In the company of others just like him and we don't get judged because his behaviour isn't considered 'normal' to parents with no experience of ASD.
Despite his difficulties he's a wonderful little boy and brings me so much joy, I just wish I could take all of his suffering away from him as I know a huge part of his tantrums result from not being able to express his emotions properly and let us know how he feels 