I submitted my tax claim forms for 2007 and 2008,they found that I had underpaid my tax credits in 2006 by 300 euro. They wiped the slate clean and did not take the money back. I dont know if they can only go back so far as to reclaim what was underpaid, but I know we can go back 4 years to submit our claims.
I underpaid my tax credits last year by 1,000 Euro and they are taking that money back over the next 2 years.
In fairness , I will never be sending in my tax credits again to make sure they dont owe me anything, I was on the winning end once, but since then I have been on the loosing end, I only do it so that I can claim medical expenses.
If the deparment was wrong with giving you benefit that you were not entitled to, they will still take it back ! It is their right, but they will do it over a period.
I would recommend anyone to check their paid tax at the end of the year, as most times from talking to other people, they always owe you, but once they pay, your tax is always sorted after that 1st claim, so I would not recommend doing it more than once.