A P800 form is HMRC's way of telling you you've paid the wrong amount of tax. It's actually just a breakdown of what tax you owe and what you've paid. In general, though, you'll only ever get one when you've paid too much or too little tax. If you haven't paid all the tax you owe, HMRC will take action to get it from you but if you've overpaid, you can get a tax refund.