If you have to pay Self-Assessment tax for the year to 5th April 2017, you must pay the outstanding amount by 31 January. HMRC accept all sorts of payment methods, but they do prefer you to pay online. However, this year they have taken away the popular option of paying by personal credit card. This is in response to new rules regarding credit card fees coming into force on 13 January (in less than two weeks). If you want to use your personal credit card you can pay prior to this deadline, but if you don’t, you’ll have to use another option. HMRC have released some guidance here.