I can't find it in any of the books lookout and scrutinizing it again I can't see why the housekeeper or Barbara didn't let the electrician in, then June appears out of nowhere when she's normally off on her travels doing charity work.
Probably the reason it wasn't in past publications is self evident as it didn't show up until 2003,or even later,before publication of CP's,Colin's or Wilkes books.
Then when the information did become available,those books which are biased,such as PH and CAL's weren't likely to give that part an airing.
Whether it's mentioned in Lomax's,I don't know. ( can't remember )
As for nobody else being in at the time,it could well have been tea-time and June had appeared on the off-chance returning from her " duties " in time to catch the meter-reader,perhaps.