Lying ex lands me in trouble with CSA

2010 leave wife, get divorced. She rings csa to make a claim, they are taking too long to get the case moving so we agree a private agreement of payment direct from me and she assures me this is fine and csa are aware, all goes well.

2012 phonecall from csa, you owe £2000+ in arreas for underpayment !! why ? because her claim has been open since 2010 and we have been trying to contact you to take the payments.

I explain we already have a private agreement but not acording to them, they say they have been tryimg to find me for 2 years, even though ex wife knew exactly where i was but told them something else.

I HAVE ALL TEXTS PRINTED FROM HER ADMITING TO LYING TO ME AND CSA forward to 2015 still owe 850 they have just increased the payments without warning and i still have ex blackmailimg me for more money every month and harrasing me to make my boss pay csa early.

Just about to call them probably to wait in a que that costs me between 10p – 50p per min. What am I supposed to do?