Letter from the CSA saying I owe more than a thousand pounds

I have always paid my csa for both my kids and at times i paid the mortgage and csa even thou i wasnt living in the house.
My daughter has fallen out with her um a number of times this year, and 3 times has come to live with me which csa have been made aware of this and every time she goes back i start at a charge of £50 set up another direct debit. On the 28th August My ex wife kicked megan, my 15 year old daughter out of the house and she came to york to live with me, I contacted the CSA straight away to let them know megan was now living with me and CSA would stop, because megan was living in York and scholing in Northallerton, it was costing me over £20 a day on transport and other costs that come with living with a teenage girl. I was always in contact with CSA but they were failing to update their records.

Megan finally went back 2 her mum on the 11th November, i told her mum i would pay a cheque at the end of the month and not going through the csa as Megan could well be back sooner than she thinks, i was also awaiting 3 months benefit which is yet to come to me, i had a call from csa saying i had 2 pay csa for the months i missed as i didnt have megan for more than 52 days which is false, my daughter was with me over 70 days,

I know my ex wife has been claiming tax credits while Megan was with me, and probably the child benefit. I had a letter from the CSA yesterday claiming over £1000 owing to my ex wife, im classing this as harrasment as ive spoken to 3 people in the csa about my circumstances, and threatning legal action if money wont be paid..Could someone please contact me and put my mind at rest.