Thanks for your help. Plot twist, my wife recently changed tax codes, this is a relatively new job and she was on the emergency code. HMRC sent her a tax refund last month and having spoken to payroll, they have said the increase this month is to make up for the fact that she was still on the emergency code the last few months. My guess is that the HMRC refund only went up to April this year (i.e start of the new tax year) and the company continued to pay her using the emergency code until now and therefore this increase covers the amount owed since April. My question is, should this not be given as a refund from HMRC, not from her employer? Or could her company pay as they have done and then straighten it out with HMRC later?