Hi NotaDaisy,
I really feel for you, as I have similar experiences, but let me just explain what helped for us.
In a nutshell, it is all about praise. Praise praise praise the good and as much as possible ignore the bad.
My DS is 8, he lives with me and my DP full time as he has no contact with his father. He has AS, he finds school very hard, and in short, can be very very defiant.
After trying evrything, we were at our wits end and someone from the special needs section recommended a token reward system. I did some research.
Now, we praise every little thing he does right, any positive behaviour we see, even the very small ones, even the things you might think "every 8 yr old should do that, why should I praise that?" and oh my god it works.
Basically kids love being praised, and when you praise good behaviour you reinforce it, and when you ignore bad behaviour you don't reinforce it.
You want to reinforce all good behaviour and basically change the child's behaviour patterns. Like training a dog.
Don't punish, don't spend hours taking about bad behaviour, discussing it afterwardas, resoning with the child.
Just praise and reward. Rewards can be anything that appeal to your DSS, a trip to the park or wsimming pool for example, and smaller things like getitng to choose what's for supper.
My DS earns card tokens, so for example he can earn tokens for doing anything I or DP ask him, cleaning his teeth, putting his clothes in the washing basket, responding without shouting, doing homework etc etc. Some things earn him a double aor tripple token, most things earn him one.
He can then exchange tokens for all sorts of things, they are his currency. He exchanges most tokens for computer time (with 1 token earning him 4 minutes) but can exchange 20 tokens for a magazine, trip to the park with DP or a lego mini figure) or 50 tokens for £1 (he doesn't get pocket money).
It works wonders, when combined with proper praise and lots of situations we set up that enable him to earn tokens (e.g preparing him in advance for soemthing we know he might have a meltdown over, reminind him that he can earn tokens for reacting in a positive way to events etc.)
The aim of the game is to allow him to earn as many tokens as he can, and as he gets good at this we can review the system and help him achieve more.