Hi T_Trim,
As you are both the beneficiaries of your late father's will, you would each be required to calculate your own Capital Gains liability.
To help you with this endeavour, there is a Capital Gains tax calculator here:
Capital Gains Tax,
Which you can each use to calculate your own gain.
If there is no gain, then keep a copy of your calculation for your records, as nothing elses needs to be done.
If Capial Gains Tax is payable, you can move on to the next screen and create a capital gains account.
You can log into the account and declare your gain, as well as pay the tax that is due.
This ideally, should be done within 60 days of the completion date, otherwise penalties and interest charges may apply.
If you are required to complete a tax return for any other reason and the disposal value is over £50000, then you must declare the disposal in your Self assessment tax return.
Thank you.