MaxB wrote:
At the London end of platform 6 at London Bridge is what appears to
be a normal 4 aspect signal, showing no light, with the label "SPAD".
Can anyone tell me if this is in use? How does it work? Are there
others? Etc..


The short answer is that it's a SPAD (Signal Passed At Danger) Indicator.

If a train goes past the associated signal (which, if memory serves, is
number L100) the top and bottom lights will display flashing red
aspects, with the middle light displaying a steady red aspect (which may
or may not have the word "STOP" across the middle).

The SPAD indicator will also have an AWS (Automatic Warning System)
(electro)magnet associated with it, which will be energised if the
indicator is activated, which will sound a warning horn in the cab (and
apply the emergency brake if not acknowledged within a certain period of
time).

HTH,

Barry