In message , bert ]
writes

Attempting to prove a negative is not positive fault finding. I would
turnoff the power during the day then go round to the shop and see if
they are running around like headless chickens wondering why their
tills have stopped working and their freezers are all defrosting.

I have already done that, and, when the power to the house was killed,
the shop lighting and computers were OK. However, the shop comprises
one room which was part of the house at one time, plus extensions to one
side and the rear. Knowing how odd [1] some of the wiring is in the
house, I would not be too surprised to find that, for example, the
sockets in that part of the shop which was originally within the house
are still fed from the house. Those sockets would not be part of usual
shop usage, and would take a while for anyone to realise they were dead.

[1] Not odd as in dangerous, but odd as in four consumer units/fuse
boxes, with not a great deal of logic, at least to me.

--
Graeme