If you are a self-assessment case, you will nee to submit a personal tax return every year.
The deadline for the submission of your self-assessment return is 31st January , following the end of the tax year that the return relates to.
For instance for the tax year 6th April 2016 to 5th April 2017, your return must be submitted no later than the 31st January 2018. This is also the deadline for the payment of any tax that may be due.
Also, be aware that the penalties being imposed by HMRC for late returns, and payments of any taxes due, are becoming increasingly more severe.