"They have had the land rented on 11mth conacre for at least 15 years but have not actively farmed it since before then. They are over 55."
They will not qualify for retirement relief; There is a persistent confusion on this issue because there is a relaxation of the rules for the purpose of the Scheme of Early Retirement from farming; Under this scheme, if you have land let on the 11mth system, you can still claim early retirement payment as if you had been farming it yourself; but this relaxation does not extend to the CGT retirement relief.
There is a separate relaxation on retirement relief which says that if you are in the Early Retirement Scheme, you will still qualify for retirement relief (even though no longer trading). I haven't checked the exact wording of the law - it might be theoretically possible for someone to get eligible for retirement relief by first going in for the early retirement scheme (though I doubt the law would be so loosely worded)