Craig,
you have my sympathy, it must be an awful situation.
But I think you need to challenge them, tell them clearly that you have made a payment (or several) - presumably you withdrew it from your account in some form, hence documented at least on your own statements? I am far from an expert on this but I would think they are required to investigate. If your wife says she hasn't received it (and presumably is lying) get her to put that in writing (even better an avidavit witnessed by a solicitor - probably not reaslistic though) - that might spook her out..? Try to get everything in writing , rather than phone conversations, then you have it available if necessary to support your case in future.
Maybe the citizens advice people can help you on the legal side? It seems grossly unfair that your ex mrs has the CSA representing her but you're left to look after your own interests.
Good luck mate.