Hello Make :) Just catching up on developments since Tuesday... I see Twunt hasn't suddenly seen the light and become a reformed character and devoted dad! 
It's really good to see you dealt with his email at lunchtime, despite having a bit of a wobble. :) Like other people have said, it's great to see you growing stronger: a week or so ago you didn't know how to reply to him at all, and now you do!
Just be a bit careful about explaining yourself too much... Firstly, you don't need to explain why you won't just drop everything and drive DD to see him this weekend - it's a ridiculous demand/request. But also, every reason you provide gives him an opportunity to poke and insult and argue and try to 'drag you in' and manipulate you...
So for instance, you say "DD is not well" and he is likely to reply some total sh*t thing along the lines of "Bring her when she's better then. Anyway I bet she's not really that ill, you're just making excuses. If she's well enough to see us at all, she's well enough to come here. And you know what - if she's ill you're obviously not looking after her very well. My mum and I can look after her just as well as you..."
And when you say "It's just not financially viable for me", he can reply with nonsense like "I bet you have more than me you money-grabbing b*tch. I pay you maintainance so you can afford it fine. And you're scrounging off the State too. Anyway, what kind of mother puts money about their child seeing her dad?"
You get the point. It'll all be total bllcks, but it will draw you in and get under your skin, and upset you. IME it really is better to stick to something much more 'basic' (even if it sounds ridiculously blunt) like "Sorry, that's not convenient. But you're welcome to come here to see her, as I said last week".
It all gets very tiresome and it's so ridiculous... You're doing really well dealing with it, and you're getting better every day. :)