KSDZ91

Hi there, This is hypothetical at the moment, but what are the steps in relation to payments on account for someone who ends up earning more, not less, than the previous year? There are instructions for reducing payments on account, but no instructions for increasing them as far as I can see. I note that HMRC's website says 'if you underpay, you'll be charged interest' - is there a way to immediately arrange payment of the difference in these circumstances? Many thanks.