Thanks for responding. And sorry I misunderstood the question.
I understand the dangers of a broken PEN conductor but the current arrangement would seem to already make it safer, subject of course to the resistance of the rod.
I knew I'd seen this discussed in a video - I finally found the right one - 20 minutes 10 seconds in:
That example is at supply source, but the same would presumably apply in at outbuilding.
Converting the outbuilding to TT is of course still an option and you might prefer it.