The P800 is the tax calculation produced by the HMRC computer when you have not paid the right amount of tax under PAYE in the previous tax year. If you normally complete a personal tax return you should not be sent a P800 tax computation, as the tax calculation is done when you submit your tax return.
Interest paid by banks and building societies has been paid without tax deducted since 6 April 2016. If you received interest with tax deducted in 2015-16, the same amount of interest, including the same amount of tax deducted, may have been carried over to your 2016-17 P800 computation. The fictional tax, which the computer assumes was deducted in 2016- 17, is shown as being repayable to you!
The P800 computation can be corrected by phoning HMRC, but you need to spot the mistake first. We’d be glad to help check any P800 tax computation you receive.