Thanks for replying, it is horribly stressful, and I feel v sorry for dsd.
In answer to Nananina - mum and dad separated when girls were 5 and 3, mum quite quickly got together with someone, who she is marrying next year. Dp had a 4 year relationship with someone which ended about 5 months before he and I got together 2 and a bit years ago. We'd been friends for a long time before that though and the kids all knew us and each other already.
I have 2 boys, the same ages as dsds, youngest ones in school together, get on fine. My oldest ds is SN with ASD, he and dsd1 are extremely fond of each other and she always behaves impeccably around him. He can raise a smile when noone else can.
I really don't agree with a lot of dps exs parenting, she does do a bit of slapping and stuff - the belt thing horrified me, but I suspect it was more of a swipe at dsds legs in the passing, rather than a sustained assault. She has slapped dsd before, and they scream and yell at each other like fishwives, but at the end of the day, if dp has tried to intervene, somehow he has always ended up the bad one, he and his ex are arguing, and dsd has gone off with her friends, returning to mums later with everything forgotten. 
Dp is just at the end of his tether I think, for a long time he wasn't being told anything. Wasn't told about meetings at school, wasn't informed about keyworkers involvement, wasn't aware of any of it.
He really wants to get it all sorted out, but he also wants a quiet life - I've tried to explain that if things don't get sorted out now, then it might lead to much more serious problems later. I've also tried to explain that at this stage, nobody is going to take the girls away...I don't know if ex genuinely believes that they might, or if it's just been for dramatic effect.
The childrens centre at the hospital is in a separate purpose built building - there is a waiting list for appointments, which dsd had, then mum cancelled.