[Solved] CMS & CB Fraud

Looking for some advice as I'm at my wits end. 

I'm currently a paying parent for 3 children (19, 17 and 16) and have been paying CMS for nearly 6 years now. My two eldest children have both left fulltime education. One the day after she turned 18 (she turns 20 in Jan) and my middle child literally a week after she started at 16 (she turns 18 in Dec) I have still had to pay CMS since then as there has apparently been no changes to the circumstances that would stop Child benefit. Last year my eldest moved in with my mother and became her carer (she was paid for this role). I contacted CMS and told them about this change in circumstance. They took nearly 5 months to decline my change request and i quote "the mother has told us that nothing needs to change".  She then forced my daughter to return home so that it appeared like nothing had changed. Myself and my wife have then put in multiple Benefit fraud claims as the mother has been claiming CB for my 2 eldest for just short of 2 years now when in reality it should have stopped and thus my CMs should have been rechecked. It is starting to put a strain on mine and my wifes relationship and finances. I was wondering are there any dads out there have been in this situation and if they have how did they go about trying to change this and if they where not successful at first how did they go about trying to go through the complaints process.

Cheers for any help or advice.