Hi Joe,
When your mother in law passed away, her share of the trust became part of her husbands assets.
As there is no Capital Gains tax liability on the transfer of assets between husband and wife, it means that the full gain from the disposal is your father in laws to declare.
The annual exempt allowance in the tax year 2023 to 2024 id £6000.
Please have a look at:
Trusts and Capital Gains: work out your tax
Thank you.