Your balancing payment for this year (2018/19) and, if applicable, an instalment for next year (2019/20) are due by 31st January 2020, along with your tax return.
If you cannot pay your tax bill, you should call HM Revenue and Customs’ (HMRC) Business Payment Support Service (BPSS). HMRC will ask you questions about your income and expenditure, assets and what you are doing to get your affairs in order. If it thinks you genuinely cannot pay your tax, HMRCy may grant you an extension to settle your taxes or agree a payment plan.
However, HMRC will not reduce the amount outstanding and will still charge you interest.
If you don't think you can afford to pay your tax bill, you should contact BPSS as quickly as possible, as it tends not to look favourably on taxpayers who have already missed the deadline. If it does grant you an extension, you will be expected to set up a direct debit. You must keep these payments up to date and pay your other taxes on time. If you don’t, HMRC will normally cancel the arrangement and start legal action.