Tax payable under self-assessment should have been paid by 31 January.
This might be a balance of tax due for the 2021/22 tax year, a payment on account for the 2022/23 tax year, or more than likely, a combination of the two. Interest runs on late payment and this interest is not tax deductible. The rate increased to % recently too. Be aware though that if payment due on 31 January is not paid within 30 days a 5% surcharge based on the amount of tax unpaid will also become payable! And it could get worse…a further 5% surcharge is levied if not paid by 31 July.
https://www.gov.uk/guidance/find-out-how-to-pay-a-debt-to-hmrc-with-a-time-to-pay-arrangement
