There cannot be any CGT to pay because the land has been gifted - no gain, no CGT, QED!
To do the thing properly, the land ought to be valued on gifting because in the unlikely event of the FIL not surviving for 7 years after the gift, some proportion of the value of the land should be declared by the executors as a gift (its a gift with reservations in HMRC speak). This could then impact whether IHT was due on the value of the land.
Ordinary decent-quality agricultural land that has no prospect of planning consent for development in worth around £5k per acre.