A recent investigation has highlighted that as many as 800,000 people could be at risk of paying tax even if their total income is below the personal allowance. In particular, the group most likely to be affected are those with multiple sources of income e.g. a wage and a pension. It's possible that in some cases the over taxation may have been going on for years. One simple way to check that you're paying the right amount of tax is by visiting the HMRC website https://gov.uk/check-income-taxcurrent-year