Your father should have ensured that there was no hazards in the field. In my view, he failed in this duty of care and is liable. Now there is also the question of was there a metal detector on the mower and was it operative? Many contractors disconnect the metal detectors as they slow down the mowing. If this is the case, then the contractor also has a liability and might only recover 50% from your father.
The question of your father's insurers is another matter. He has a duty to ensure that he takes reasonable precautions to prevent loss. Surely the field was checked since the gate was placed against the gap? If it fell, then the gap should have been obvious. Insurers might not cover him.