Anyone who pays UK income tax one year can be required to make tax payments on account in the following year in addition to any tax deducted at source via PAYE, the standard pay-as-you-earn system. If the payment on account would be less than £1,000 or if the "relevant" amount is less than 20% of the "assessed" amount, then you are exempted from making payments on account. Not many people know that.
Those who do know that sometimes want to apply to have their payments on account reduced for which HMRC, Her Majesty's Revenue and Customs, kindly provide a form SA303.
There's a copy of the SA303 still available in the National Archives but that's all old hat now. These days, in the new world, you want to go to Self Assessment: claim to reduce payments on account (SA303) on the web, where you're presented with a choice:
GOV.UK Verify (RIP) account and fill in the SA303 on-line or ...
... what? You can choose the the Postal form option in which case two clicks later you're asked to complete the following form on screen:
It's your choice.