Do you know the named social worker for your 9 year olds case? I would suggest that you put your concerns to them, a lot of people are afraid of social workers and what they can do and don't always think about what they can actually offer. Plus, I know you read horror stories in the paper but most social workers are pretty savvy and have seen and can see through a lot more than they will necessarily let on. They are trying to act in the best interests of the child and if your partner has been non-co-operative, them rolling in there, shouting the odds and telling her she's a bit of a crap mum ain't really going to make her more co-operative. I would suggest that you let the socail worker know of your concerns in a calm and reasonable manner without making allegations. You should also be prepared to answer why you declined additional maintenance (not saying you should, just that you might have to explain it).
Also try not to turn it in to a you v the ex thing. It is the welfare of the child that is obviously paramount and if the social workers think that you are both more willing to score points off the other rather than concentrating on the child, then that will raise some alarm bells. Again, I'm not saying this is what you are doing, just advising how things can be perceived.