Hi Jonathan Matthews,
The price of the property will be split into two.
Your 50% at the time of acquiring the property in 2011 and 50% of the market value at the time you acquired the other 50% in 2016.
When added together you will obtain the acquisition cost.
As the property was your main residence, you can claim private residence relief and this may cover all or most of any gain arising from its disposal. Please have a look at the guidance here:
Tax when you sell your home
There is a calculator to help you work out your gain here:
Capital Gains Tax.
If tax is payable, you have 60 days from the completion date to report and pay the capital gains tax, to avoid penalties and interest charges.
You can create a capital gains account and report and pay the capital gain online at above link.
Thank you.