'If my son, Jeremy Bamber is, in the opinion of the trustees, farming with me at the date of my death, or the date of the death of my wife, whichever is the later, and are resonably satisfied that he intends to carry on farming thereafter, then the remainder of my residuary estate should go to my son'.
Don't forget the rest Adam, otherwise some might think you're withholding information. Also, you need to quote it properly. You missed out the bit about Sheila getting £10,000.
"If the Trustees are not satisfied that my son will continue in farming as aforesaid (the decision being in their absolute discretion) then my residuary estate shall be divided between my children in equal shares."
As you can see, Sheila would have got only £10,000 if Jeremy continued to farm, but if he did not, she would half and
he would get half - he wouldn't be disinherited.
It's an interesting Will. How would the Trustees make that judgment? What if Nevill died naturally and Jeremy carried on farming until it had all been settled and then stopped? There was no time limit imposed.