Simple Assessment not received

As of early December, I have not yet received my Simple Assessment for tax year 23-34. I estimate I may owe about £1k in tax, but cannot pay it as there is not yet a 'debt' on the system to pay it against. Should I just keep waiting for the Simple Assessment to arrive, or should I submit a 'voluntary' Self Assessment by 31st January? The HMRC website is silent on what to do if you are meant to get a Simple Assessment, but have not received one.

Hi,
You should wait until you receive your Simple Assessment for the tax year 2023/2024 -
Pay your Simple Assessment tax bill.
Thank you.