The reason you cannot find any guidance on increasing your payment on account is because you are not required to do so.
If your profits are rising this “shortfall” will be picked up in the final balancing payment, but with no interest applied. So, in answer to your question, pay the amounts shown in your self-assessment when due and you will not be charged interest.
Your question also gives me the opportunity to comment on some related issues.
You can receive an unpleasant surprise the first time you are caught for these payments. This is because on the first time the tax is demanded, on January 31, it will be for both tax on your actual profits for the year plus half as much again for the first payment on account for the following year.?
This can be particularly painful if you are not expecting it and have been
slow in submitting your self-assessment
leaving little time to find the money.
You can avoid being brought within the payments on account rules if in your previous self-assessment, the remaining liability was less than £1,000. In addition, it will not apply where you have paid more than 80pc of the tax liability for the previous year, typically because you were also being
taxed under PAYE on your earnings
or private pension.?
If you are in this position and concerned that you might just slip over this threshold you might consider making an extra pension contribution, or possibly a gift aid payment if you pay at higher rates, to keep below it.
Once in the system, the payments you have made on account can be offset against the next 31 January liability.?
However, I don’t think the layout of the online tax calculation is helpful in explaining this. The calculation shows the amount of tax due for the year plus the first payment on account due, but it does not automatically offset the two on account payments already made.?
There is a note at the bottom for the calculation saying: “This amount does not take into account any 2022-23 payments on account you may have already made”. HMRC has this information and it would be helpful to show these payments as a deduction.?