Overdue Simple Assessment

Dazed_and_C0nfused

Saver73 said:

thank you for correcting me @Dazed_and_C0nfused I had a notification from HMRC recently saying I owed tax (an underpayment for tax year 24/25) and I was given the option of having my tax code amended or to pay the amount in full. The OP's situation sounded similar. 

You will have had a P800.  Which is essentially the same calculation, but is issued when HMRC intend to collect the tax via your tax code.

But you can pay it voluntarily if you would prefer to get it out of the way and not have your tax code messed about with.