Underpayments (PAYE)
From 2017/18 HMRC is planning to collect potential underpayments in the tax year that they arise rather than in the following tax year as at present.
Actual tax P800 underpayments for 2016/17 will also be collected in 2017/18. In general then the payment of tax will be advanced.
Some safeguards will be in place to restrict the amount of tax that can be collected this way.