CSA slap a bill on me to compensate for their mistake

I transferred from the csa to cma on 21.09.17. Have just received a letter from the csa stating I owe them over £5000 I have been paying £317 per month since 2002. I had a deduction of earnings off my pay every month to cover this. They said they assessed my account in 2012 and stated that I should have been paying £97 per week but never increased my deduction of earnings to take this into consideration. They now say I have 14 days to appeal this otherwise it will be handed to the cma who will add it to my arrangement. Are they in the wrong as never increased deduction of earnings. I understand that if I have to pay it, it will be over 2 years. Can anybody give me any advice as I am desperate.