Self assessment and underpaid tax

Cazzie

Hi All, I am currently dealing with a client and self assessment. When I have done the self assessment they have underpaid tax in previous years through PAYE, however when they have spoken to HMRC today they have said that the underpaid tax has been taken and paid off and she doesn't actually owe this. HMRC have told them that on the tax return, you need to tick a box that is , 'yes for previous years under end of year adjustment '- second to last sections-Tax You Owe. I cannot see what she means, can anyone help me and tell me where I would find this to reduce the tax that they owe to cancel out what they have taken from their PAYE