I have an update anyway - spoke to my neighbour about an hour ago and
mentioned this thread. He told me that after a visit from some security
people they said they thought likely that it dated from WWII. The BAC
works at Filton were of course involved in producing the Bristol
Blenheim and Bristol Beaufighters as well as repairing damaged aircraft
so it is quite possible that the aerial was something to do with
espionage then. Who knows.

What makes you think it was a transmitting aerial??
It was very common in the early days of radio to have external aerials
for receivers, quite often down the garden, but also around a room,
often on the picture rail. It just sounds like a 'neat' installation
that was discovered.

Jeff