This thread seems to have expanded a bit from your inital query, and I wanted to pick up on some of the things you've said since I was on here earlier.
"Regardless of whether my ex has contact she is still his daughter and he should support her financially - which isn't possible on £14 a week."
But if that's 15% of his net earnings, that's all he needs to pay according to the CSA, therefore he is supporting her financially. If you are struggling to cope you may need to look into budgeting more - change utilities to cheaper ones, cut any luxuries, drop down a brand at the supermarket. I'm probably teaching you to suck eggs here, but there's a lot of good advice on Money Saving Expert.com.
"Why should it matter if his wife was helping to pay the maintenance before? Like I said they could afford it or it would have been reduced a long time ago."
No it wouldn't have been reduced because you had a private arrangement. How do you know they could afford it? How do you know they aren't struggling? I have a friend who's DH pays maintenance to his son from a previous relationship. He pays quite a lot, but he has two children with his new wife and although she also works, they are struggling to make the payments. So you don't really know what's going on behind closed doors.
"The reason our split was messy is because he left me for her. I was with him for 6 years then he left and in less than a year had married her. Yes I was upset and I didn't want my daughter to see her father with someone else. I also didn't want to have to explain why he married someone else and not her mother. That woman took my partner and my daughter's father so she should imo be paying for that."
Oh dear, you sound very bitter tbh. Kind of understandable under the circumstances but it was years ago and you have to let it go and move on. I say this with all sincerity and the best of intentions, but have you thought about counselling? It can work wonders when dealing with emotive issues like this (trust me, I know!). And sorry, but no, his new wife shouldn't be paying for what happened. She didn't hold a gun at his head, he left of his own free will. As you'll have read in my first post to you, similar happened to me so I'm not without sympathy but you can't lay the blame at her door, then punish your ex by denying him contact. It's not on.
"Oh and I do think his new wife has obligations towards my daughter - she was the OW and caused our split."
As I said above, he chose to leave and she has no obligations to your daughter whatsoever.
"He did get solicitors involved but he never took it to court so he couldn't have cared that much about contact or he would have tried harder. He is as much to blame for the non-contact as I am."
Yes perhaps he could have tried harder but as someone said above, maybe he didn't want to prolong the hurt. Also, it should never have got to the point where he had to approach a solicitor. Your statement that he's as much to blame for the non-contact is just your way of transferring the guilt and your argument doesn't hold water.
"I don't think my dd would want to see him anyway its been too long and she never seems bothered about it."
You're putting words in her mouth. Have you asked her? I mean, actually said that if she wants to see her dad she can, and asked her if she'd like to see him? She might think you'll be upset or angry with her if she expresses an interest in seeing him - you come across as very angry about it on here, so no doubt at times this is vocalised in real life too - so this may put her off mentioning it. And the only reason it's been too long is because you let it get to this point. Bear in mind that this may bite you on the bum when she is old enough to find him by herself and realises you prevented contact.
"Softkittywarmkitty - you say the CSA will be closing all the cases - does that mean I can go back to the private arrangement then or do the CSA assessments still stand and are just paid privately ?"
No, you will only go back to the private arrangement if your ex agrees, and by the sounds of it he won't. So you'll need to pay to reapply and they'll start looking into your case from scratch. Except you'll receive even less money, because the CSA will then take a percentage of your maintenance as their fee.
"I think it is unfair that he has his wife bringing in money while I am left struggling. I gave up work to be a sahm when my eldest child (not his) was born and I haven't worked for 16 years so the chances of getting a job now are slim to zero."
She has her own business and that must be very stressful, so she probably works very hard and deserves that money. You chose to give up work. Don't begrudge someone who's worked hard their financial reward. You get out what you put in. And on that note, depending on what kind of work you fancy doing, but to get experience of retail maybe you could volunteer in a charity shop a couple of days a week, or volunteer in a school? At the same time you could do an OU course or course at the local college. If you don't work, the fees will probably be paid for you.
"my other ex can't increase the amount he pays."
Why should he? Your DD isn't his so she's not his responsibility. Also don't forget that if you have one son who's 16, his dad won't legally have to pay maintenance once he's out of education, so you may need to prepare for a further drop in income in the next few years.
"My children need me there when they come home from school and I have no skills so getting a job would be very hard after all this time. I could only do part time and I'd need all the holidays off because of the children."
Rubbish. I'm a lone parent, and when my DS was 6 months old I went back to work full time. DS went to nursery 3 days per week and to my parents 2 days. If you work you will get working tax credits towards your childcare. Now my DS is at school and I work part time 3 full days per week, and on those days my parents do the school run. I don't get the holidays off but I'm lucky as my parents have him the days I'm working in the holidays. But if they didn't, I'd probably use a childminder. Again, working tax credits would help with the costs of this. Realistically, how many jobs take place in school hours with school holidays off? Not many, and those that do are rare as rocking horse shit as all mothers (and some fathers, no doubt) would ideally like to work those hours. But we don't live in an ideal world and I think you need to expand your horizons a bit rather than dismiss any job that's not strictly within the school timetable.
I'm assuming as you don't work that you're on income support? Not sure if you're aware, but new rules are coming in (I think in 2012) where a lone parent whose youngest child is 5 or above will be moved from IS to JSA and expected to look for work. If they don't find work within 12 months, their Housing Benefit will be cut by 10% (not sure if you receive HB?). With all this in mind, and bearing in mind your kids are getting older so won't be entitled to CB and maintenance once they are in their mid/late teens, you may want to look into preparing yourself to get a job sooner rather than later.