Hi flibulating_meerkat,
You will need to obtain the market value of the property at the time the property is gifted to your children.
This will be the disposal value.
Two calculations will be required. The first is your 40%.
You would use your purchase price and costs and deduct this from 40% of the market value at the time of disposal.
For the other calculation, you would use the market value at the time of inheritance plus costs and deduct this from the remaining 60% of the market value.
Both gains should be added together.
There is a calculator here that will help you work out the gains:
Tax when you sell your home
Thank you.