HMRC penalties for late paying of self assessment due 31 Jan 2025. Do they compound?

[lander]

Say my SA bill is £250 &I don’t pay on 31 Jan 2025. There is a penalty of £100.

On 28 Feb, I still haven’t paid, will I owe 350(1+.07/12)=350(1.005833)=352.04 ?

If I still haven’t paid by 31 March, will this sum be compounded to 350(1.005833)2 =354.09... to n mnths?

HMRC are not very clear how they do the calculation.

[bd6759]

Interest isn’t charged on interest.

[bd6759]

The late payment penalty won’t be £100.

https://www.gov.uk/estimate-self-assessment-penalties/y/2022-23