Not wishing to alarm Grant any more, but one thing I have not seen
mentioned yet is performing an insulation resistance check on the
current circuit either. Since it sounds as if the place is wired with
"older" cables, that could be a another potential problem area.

Unlikely, though. PVC is pretty durable.

However, given that only one circuit was used for the entire house, it is
possible that it has been running close to design current at various times,
which may have degraded the insulation more than it might otherwise. Apart
from this, I'd expect PVC installed even 50 years ago to be in serviceable
condition.

Obviously, a periodic test should really involve an insulation test. And it
seems that the entire installation could benefit from one.

Christian.