what have I overlooked before seeking an assessment?

You might find it helpful to compare the traits that you've already identified with those detailed in these resources, and to note down any additional things that you recognise in yourself:

NAS - What is autism?

NAS - Signs that a child or adult may be autistic - includes over 60 signs.

There's also some advice here, from the diagnosis hub:

NAS - What can I do while waiting for an autism assessment?

NAS - Criteria and tools used in an autism assessment

Just by way of a kindly-intended forewarning, I'm sorry to say that the imposter feeling that you mentioned can return (and recur) even after diagnosis. You might find these resources helpful on that front:

NAS - How will I feel after receiving an autism diagnosis - includes an example of someone else discussing their experience of imposter syndrome. 

Psychology Today - Why do I feel like an imposter after my autism diagnosis? - 6 ways to overcome imposter syndrome after a later-in-life autism diagnosis

Authentically Emily - Autistic imposter syndrome

Thanks, Bunny. There's nothing documentary from childhood at all except exam certificates. I've already gone through the various categories and matched things I remember to them across my life. Regretfully for me, that exercise closely resembles the internal job application process at work, which, of course, raises anxieties. Still, I can work through that. 

Yeah, imposter syndrome is a pain. I realised today that I can easily find reasons to discard an autism assessment and despaired at theis self-defeating cycle. My post is my attempt to get past that. :)

I know that some other people here have talked about their own childhood recollections having been accepted.

My assessors did get input from my parents, and from my school reports, but I have a terrible memory myself and can't remember much at all from my childhood. So I asked my GP for my full medical records, to see if there was anything useful there. 

They were helpful both in helping me to remember various things, and in containing evidence themselves (for example, of sensory sensitivity as a child, to an extent that prevented me from being examined properly).

For me, it was important to get copies of my full records for me to review myself, because I knew what kinds of things I was looking for (being armed with a good understanding of autism).

My GP surgery had initially misunderstood my records request, and someone there had gone through them and - based on their more limited understanding - sent me copies only of things that related to my mental health issues.

Info for accessing them in Scotland, in case you'd like to do the same:

NHS Inform (Scotland) - Health Records > Accessing your health records

Thanks, Martin. I've covered those in my notes. One thing that occurred to me a couple of days ago was being given speech training for over three years from the age of 6/7 on the grounds that my teachers couldn't understand my thick Lancastrian accent. Looking back on it, that explanation makes no sense to me because I was 3 when we moved to London and my parents/grandparents were from different parts of England, none with thick accents. 

Recollections of early childhood such as: liking to be alone, taking things said in jest literally, not calling at other children's houses to play (even if they came to yours), not liking parties, not replying to questions asked by adults that you did not know well or having to be prompted by a parent to answer, anxiety about school etc. all these sorts of things are useful pointers to autism in childhood.

For a diagnosis to be given there needs to be some indication of autistic traits in early childhood, but your own recollections should be valid to cover this requirement. I had periods of selective mutism after starting infant school, which seemed to be all that was required for my diagnosis.

Recollections of early childhood such as: liking to be alone, taking things said in jest literally, not calling at other children's houses to play (even if they came to yours), not liking parties, not replying to questions asked by adults that you did not know well or having to be prompted by a parent to answer, anxiety about school etc. all these sorts of things are useful pointers to autism in childhood.

For a diagnosis to be given there needs to be some indication of autistic traits in early childhood, but your own recollections should be valid to cover this requirement. I had periods of selective mutism after starting infant school, which seemed to be all that was required for my diagnosis.

Thanks, Martin. I've covered those in my notes. One thing that occurred to me a couple of days ago was being given speech training for over three years from the age of 6/7 on the grounds that my teachers couldn't understand my thick Lancastrian accent. Looking back on it, that explanation makes no sense to me because I was 3 when we moved to London and my parents/grandparents were from different parts of England, none with thick accents.