Historical Tax Credit overpayment

Hi
I received a letter from the HMRC today stating that I owed money for an overpayment in Tax credits in 2005. It stated that they had written to me about this and not heard from me.I called them and they said they had written to me in 2010 about the overpayment. I have no recollection of this letter but I may have missed it due to alot of upheaval going on in that year. I havent recieved Tax credits since 2010.

When I asked why it had taken 5 years originally and then another 9 years to contact me about this they didnt have an answer but said that they thought there had been some decision made quite recently that meant that they could reopen past accounts.
They suggested that the overpayment in 2005 was due to a change in household income.

I am wondering whether I have a basis for appeal given the length of time this has been laying dormant. I am not in a position to pay the money and it has come as a shock to recieve it after all these years.