Self Assessment - Payment on Account for the next tax year

I've completed SAs for many years. I always like to settle up by making a one-off payment to HMRC instead of having my tax code altered. For the last tax year 23/24, the SA tax calculation is showing a due amount "X" which is normal. BUT it is also showing an amount "Y" for "1st payment on account for 2024-25 due 31 January 2025". The "Y" value is half of the "X" value. Please can you explain the "1st payment on account for the 2024-25" tax year"? Have I crossed a threshold? Has something changed? Thank you I am a pensioner with untaxed UK interest and some dividend income which I have set to £1 because I'm awaiting the figures from my platform provider.

Hi Bluebell74 Yes,
When your Self Assessment is processed if the balance due is more than £1000 and you pay less than 80% of your tax at source then payments on account are introduced for the next tax year:
Please see guidance here:
Understand your Self Assessment tax bill
Thank you.