When do I need to make payments on account?

Payments on account are advance payments towards next year's tax bill, generally required if your last Self-Assessment bill was over £1,000 and less than 80% of your tax was collected at source. They're due in two instalments: 31 January (alongside your balancing payment for the previous year) and 31 July, each usually half of your prior year's tax bill. If you expect this year's profit to be lower, you can ask HMRC to reduce your payments on account, but be careful not to reduce them too far or interest can apply.