If you have moved house in the past few years and are not self-employed or unemployed, sit up and take note.
When you move house, although you may tell your employer your new address, it does not tell the Inland Revenue. And even if it did, the Revenue doesn't take notice as it can only change your details if you inform it.
The reason why this is bolt-upright urgent is because more than five million letters (never emails, they are scams) are being sent out telling people they have overpaid tax in the past two years and are due it back. So keep an eye on the post.
But if the Revenue does not know your address, you can inform it via www.hmrc.gov.uk. Alternatively, call your local office. Plus, although it is two years worth of tax that is being looked into, you may have been mischarged for far longer. You can check if you have paid too much at www.taxcodechecker.com