Hi,
You will have two calculations to perform. The first is your share using the acquistion cost in 1987. The second is the inherited share, based on the probate value of the property.
Each calculation can have your share of the costs applied such as solicitor's fees, estate agent fees and so on, and the difference is the gain or loss.
You can use the calculator to help you calculate any gains, as well as report and pay the Capital Gains Tax due here:
Tax when you sell property
You can also see guidance on Private Residence Relief here:
Private Residence Relief (Self Assessment helpsheet HS283)
Thank you.