That is reasonable!!
My OH should only be paying about £40 a week to his ex, but as he had the DNA test (even tho he was 95% sure he was the dad) he had some back payments to make and so he pays £50 a week, thinking about it, his back payment is a hell of a lot for a month or two :S
But anyway, if she doesn't agree without going through CSA, then you OH will pay £5 a week/fortnight if he is claiming beneifts while out of work, so don't think they will put a back payment on it, and you could still make it up to the £40 a week.
Tho id keep evidence of you paying her money so that she can't claim that he hasn't paid a penny towards the child.
Just to point out, iv only read first post, not the whole thread, that's just basicly what we was told when OH's EX put in for CSA.