If he's not on the BC who need to see the social worker who will ask for a stat dec to say you don't know who the father is. Unfortunately if you know his name, or he's on the BC and you DN'T claim CS, they'll usually not pay you any more than the the minimum amount.

You'd probably have gone through all this anyway, seeing as your DD is 18 months old. We went through it before we were approved for parenting payment just after Jazz was born (last year, when we were still seen as single). They tend to be on top of things like overpayment when you're not eligible etc.