I believe he should pay you 15% of his net earnings for one child, so this is what you should get. If £30 is much less than that then you'll have to appeal. Maybe write to your MP. You do realise that the CSA will soon be charging for their service don't you? That means your case will be closed and you'll have to pay up to £100 to reapply, then they'll also charge both you and your ex a monthly fee. Imo you'd have been better off sticking with the private agreement. £300 per month is a huge amount of money. I get £12.50 per month. Try coping on that.
Also fwiw I don't feel you had the right to stop him seeing his child when you split up, unless there was a genuine reason such as dv, drugs or abuse - in which case a court would have restricted contact or only allowed supervised contact. If your DD has just started secondary school and you split up 8 yrs ago, I'm assuming she was 3 or 4 and was at an age where she could have expressed an opinion about seeing him. Did your DD want to see him? Did you not think it would have been 'in her best interests' to keep as much contact as possible with her dad to keep her life as stable as possible, rather than to prevent her from seeing her altogether? And as for you not wanting your ex's new wife to have contact with your daughter, that's not your call to make. She's clearly not some fly by night girlfriend, it's his wife and they have children - your DDs half siblings which, because of your decision, she does not see . If you've ever had/have a partner over the last 8yrs have they met your DD? What's the difference between her meeting a DP of yours and meeting her dad's wife? Same thing, isn't it?
I'm sorry to be harsh but my ex has never seen my DS, despite my best efforts to get him to, and I would have loved for my DS to have a relationship with his dad. I know this isn't strictly what this thread is about but it gets on my nerves when women play god with their children's lives, usually just to get back at their ex after a 'messy' split, or because they are jealous and bitter that their ex has moved on. Two things I really detest are women who stop fathers from seeing their kids and fathers who don't take any responsibility for their kids. I know that contact and maintenance aren't (and shouldn't be) linked but part of me feels that this business with the money is karma. Sorry.
I agree with truckulente in that if you want to improve the financial situation, I'd suggest reintroducing contact. Then your DD can get to know her dad and half siblings (yes, and her stepmum!), and as time progresses her dad may be more willing to discuss increasing maintenance, so this would benefit both you and your DD in the long run. Also her dad would naturally start to pick up the costs when she's with him, which would take some of the financial pressure off you. Is reintroducing contact something you'd consider?