When your total liability for a tax year (eg. 2018-19) is more than £1,000 and it is not collected by PAYE you are then required to make 2 x 50% payment on account towards the following tax year (eg 2019-20). These payments are due on 31.01.2020 and 31.07.2020. This then means if you had identical income for 2019-20 you will have paid your full income tax via payments on account and the payment due on 31.01.2021 will be the 50% payment on account for 2020-21 only.
If the two 50% payments on account don’t cover the full 2019-20 tax liability then on 31.01.2021 then there will be a balancing payment of the difference as well as the 1st 2020-21 50% payment on account.
More information is available here: