Thank you all this really helps, I'll start asking about the ADHD again. There's no issues at home, apart from this. He's always been like it. He spoke really early as a baby/toddler and when he was tiny if I said to him "that's naughty" he'd say "you're naughty" and blow a raspberry in a really annoyed way!
I asked his teacher a couple of years ago, when they'd told me about some of his behaviour if they thought this was "normal" or it could be ADHD or similar. She said she's not medical and she wouldn't know. That's when I paid for the private psych appointment. They could do ADHD assessments, they were ridiculously expensive, like 1 or 2 thousand pounds, but I would have paid it if they'd said it was likely. As they said he was just hard work I just went away with parenting tips.
The school did refer us, not him but us as a family to the family help team for problem families, because he kept telling them stuff like he hadn't had breakfast and was hungry so they had to feed him. He's never, ever gone to school without breakfast or hungry. He also used to cry at lunch and say he didn't like the school dinner when it was stuff that he likes and eats all the time, so we gave him packed lunches, and then when he'd say he hadn't had breakfast they'd give him something from his lunch. He also told them that his punishment at home is getting slapped round the face! To be fair, I have smacked his bum or the back of his hand immediately after yanking him off of his siblings, especially when hurting the toddler, but even that we (with dh) spoke about the fact that didn't actually help as a parenting strategy, and so now he gets screen time taken away, or has to stop the fun activity he's ruining, or go to his room. I bought a positive parenting course online and that's great for strategies, but it's just so relentless. He never learns. He never listens. He can't be told something without saying or doing something back.
We've followed the family help team advice, they spoke to the kids for maybe 45 minutes as well to get to see them and the dynamic but they said our strategies were pretty good generally, and gave some tips, and they've referred us on to a "lesser problem" team to close the case really.
They tried a reward chart at school but almost straight away he had an absolute meltdown all afternoon when they wouldn't give him a sticker because he'd been really naughty.
The person that said about moaning about fun stuff, he does this too. He just ruins every fun thing for everyone every time moaning or being naughty. He asks questions just so he can complain whatever the answer. Even if you say yes, it's not good enough or soon enough. We head out to take them bowling, picnic whatever he cries, doesn't want to go, can't manage to put his own velcro/slip on shoes on, at all or on the right feet. It's just sucked all the joy out of life.
To be fair he has improved quite a lot since lockdown, because we're not asking him to do anything, certainly not in a hurry. If he needs to put school shoes on, you've got 10 minutes or so. In lockdown, I'll ask him so many times to put pants on and clean his teeth and eventually it'll get done each day! He does have sensory issues too.