Hi thanks so much for all your replies, they are really appreciated. To answer a few of your questions.
They are in contact through other routes, Skype and e-mail, in fact ex has only just re-added DD as a friend on FB after the last lot of bollocks his wife posted about me. I did reply to this and they had the cheek to report the post as I was so nasty to her. Completely ignoring the fact that she had offered to give her fb friends my address so they could shove me under a bus!
I did see a solicitor about this who advised me to go to the police, but a friend in the force didn't think it would go anywhere as the post on FB was quite old before I saw it so not an immediate threat.
DD's see their dad every other wknd, he does occasionally have them in the summer hols for a few days at a time, he never requests any further contact. Lately both DD's have on occasion decided not to go for contact wknds, not ideal I know but as they are 13 and 10 and there is no formal agreement I think it's up to them?
Youngest DD especially doesn't like going by herself, she doesn't get on with stepmum at all, as she has bitched about me to her a lot in the past. Ex generally works nights at the wknd so he's not really missing out.
As for the FB account I really don't know, thinking about it, it seems odd that he has chosen now to post this knowing in the past I have always managed to find out what he's said? I'm not sure if eldest DD would've seen the post, she was with him at the time he posted it and tbh it was the comments posted after rather than the post that pissed me off more.
I think the thing that bugs me most is that I cannot fathom why he does this shit, he can see the DD' s as much as he and they want, he has other contact, he's always boasting about how wonderful they are on FB so what's the problem?
I just feel that whatever I do it's wrong,
Sorry for the novel, it's been cathartic to get it all out.