This is something which has long caused me some concern. If, for
example, the house is earthed by an earth rod (rural setting), and has
a phase to earth fault such that said fault is not sufficient to trip
the circuit's mcb, and is sufficient to bring the earth wiring, bonded
services and earth rod upto mains voltage - I've seen this happen -

I would say that the chances of electrocution in this failure mode is very
rare.

Firstly, it presumes that the house TT earth RCD has failed. Secondly, the
fault condition within the house after this failure is so severe (all metal
appliances are now live), that the minor risk of an insulated gland being
live behind its sheath in the shed is of no significance whatsoever.

Christian.