What if child maintenance payments aren't being spent on the child?

Secure the services of an attorney. Bring all bills and/or receipts for what you've shelled out. See if the attorney will work through the courts to still maintain the arrangement you have OR cease what you're doing now and make the payments. Its up to HER to show she's using the monies for the support of the child although be forewarned this is not carved in stone. She can use the money for whatever she likes as long as the kid is clothed and fed.