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Assuming you exchanged contracts to sell after 5 April 2016 and you don't live outside the UK, then you will need to complete a tax return for the 2016/17 tax year which does not end until 5 April 2017. If you complete it online, you will have until 31 January 2018 to submit it to HMRC and any tax due will be payable on 31 January 2018 also. Complete a form SA1 to register for self-assessment if you have never been registered before. If the property was ever your main home, then HS283 may be of assistance. There is information on Capital Gains Tax here and here.
I hope this helps but let me know if you have any further questions.