In article .com,
Osprey wrote:
[snip]
Anybody any suggestions of something I can inject 'into' the
cable, and how I can trace it ?

( The cable is a foil screened CAT5e )

Perhaps a nasty spikey/square wave, injected
into the cable between screen and Earth. The
spikey/square waveform gives a broadband noise
source. Then look for the far end of the 'aerial'
with a transistor AM receiver.

Perhaps generate the nasty spikes with an old
fashioned buzzer, either bought or made up with
a relay and dc supply.

ASCII art follows, view with fixed-width font
and no automatic wrapping.

DC+ ----+---------+
| |
_|_ )|
Diode /_\ )| Relay Coil
| )|
| )|
| |
+---------+-----------Cat5e Screen
|
+
/ Relay, normally closed contact,
+ (opens when relay is energised).
|
DC- --------------+---+
__|__Earth
/////

There will be a bigger signal without the diode,
but the relay contacts will be ruined without it.

--
Tony Williams.