Thanks for all the responses.
I tried to keep my original post short and to the point, really, and when I described DS1 as 'shy', 'stubborn' and possibly a 'late developer' I guess I missed out a lot of other stuff.
Other issues include: very bright in terms of learning numbers and letters, fantastic memory (he once wowed a circle of doctors around his bed as he read a book from memory while in hospital after an asthma attack), limited imaginative play, can get upset by changes to routine, gets obsessed with certain TV programmes/DVDs for weeks on end, only just started drawing and trying to write his name, usually goes in another room when we have visitors (although often slowly comes around once they've been here for a while), physically a bit awkward (best way I can describe it), was late to jump, very disobedient at times, seemingly no desire to please or be like his peers (e.g. if you say 'you are nearly 5 now, you should be feeding yourself like your schoolfriends', this has no impact whatsoever), can undress but needs help and constant reminders with dressing, finds it hard to sit still... so you see what I mean!
However, he is improving in some of these areas - he is now eating a good, balanced diet (although we sometimes have to feed him still), he potty trained well aged exactly 3 after a few weeks of a 'battle of wills', he sleeps very well (needs lots of sleep because he is so physically anxious I think), he enjoys going to playgrounds, using his scooter, getting better on bike (with stabilisers), likes messy stuff like sandpits, glueing, can now cut out with scissors, has a laugh being thrown around on the bed with his dad, sometimes he now understands SOME changes to SOME routines as long as it's explained to him, he is a bit more interested in other children but he often talks with them in an argumentative way or just plays superficially in games like chasing etc
He also doesn't play with toys much apart from laptops, computer games, etc but he does like books and occasionally has 10 minutes playing with Lego.
I was shy as a child myself and am still quite introverted at times. I like the way I am - I like it that I think a lot and I wouldn't want to be any other way. It is not the possible shyness that I am concerned about - if he's shy, sensitive, etc, then so be it. Also, I don't care what his interests are - if he's not sporty then fine. I just want him to be happy.
As he is still young and I don't know if he has borderline anything or if it is just an extreme personality, I feel like I am going around in circles when I worry about it.
I think I have done the right thing by starting the diagnosis process as it may be that at some point he'll need help and we'll be closer to getting it.
At the moment, he has surprised us all by quite liking school (there was a wobble during the first week). I have not yet mentioned my concern to the teacher as I want to see how he settles in without colouring the teacher's own judgement. However, she has put little signs on the whiteboard with pictures of what is going to happen during the day. She said these were for DS1 and some of the others. So, she may have an inkling...
Ridiculously long post - thanks again for all your responses. I'd be grateful for any more advice / your personal experiences.