Under the self-assessment system, a taxpayer is required to make payments on account – advance payments towards the eventual tax and National Insurance liability – where the previous year’s self-assessment bill was £1,000 or more, unless more than 80% of the tax liability is deducted at source
The deadline for filing the 2017/18 self-assessment tax return of 31 January 2019 has now passed. You filed your return on time and paid the tax that you thought was due, but you know realise that you have made a mistake.
There are various reasons why you may have paid more tax than you needed to for a tax year. For example, if you only worked at the start of the year, you may not have received all of your personal allowance.