In article , Pete
writes
Is it possible to power an LED from a light switch (live only connections,
and possible earth) so an illuminated LED can indicate if the light is on or
off?
Yup,

fixed width font

LED
----RES180R---||----
| |
SWITCHED-FEED----||--||--||--||------LAMP
| |
---------||---------

--||-- = DIODE

A bit heavy on component count but lights evenly independent of lamp
load, choose diodes to match full lamp current. LED is low current
(5mA).

All parts are live, including the LED. For safety don't poke the led
thro the panel, use a plastic holder.

Extra components would be required to meet BS/VDE safety.
--
fred