Since July 2011, HM Revenue and Customs (HMRC) have been sending out P800 Tax Calculation documents relating to the 2010/11 tax year. You’ll typically get a P800 if you have paid too much or too little tax during the tax year. Keep reading to find out what you should do if you’ve received a P800 form.MRC prioritise people who paid too much tax during the previous tax year in order that they get their money back quickly. If you have paid too much tax a cheque will automatically be sent to you within 14 working days of the date on your P800 Tax Calculation.