For this tax year, HMRC expect you to make a payment in Jan 20 for this tax year as a contribution to tax as most of the tax year has passed by (I.e. April 19 to Jan 20).
Then in Jul 20 they expect that the full tax bill is paid. However, most people do not submit their tax returns until after Jul 20. Once HMRC receives the tax return (after Jul 20) they reperform the calculations for tax due, and then they allow individuals to pay a final payment in Jan 21. Needless to say If your tax bill is lower than your total payments then obviously you will be due a tax refund.
It can be quite confusing, but what this means that if you think about the January 21 payment alone, HMRC will assume you are not only settling your previous tax years bill for19-20 (if any), but also expecting a payment on account for the current tax year 20-21.
Official Entity Name AGR Accountancy LLP
Raj@agraccounts.com