If you had any income from self-employment or any capital gains for the year ended 5 April 2015, and you intend to submit a paper tax return, then the deadline is 31 October 2015. The same tax return can be submitted electronically, in which case the deadline is 31 January 2016. This is also the deadline for any accountant in public practice who will prepare any accounts and submit an electronic tax return for you for a fee. It is advisable to see an accountant if you have left things this long.