coughed up some electrons that declared:

What the OP wants is a "panel neon", e.g.
http://www.maplin.co.uk/Search.aspx?...neon&source=15 or
LED: http://www.maplin.co.uk/Search.aspx?...0led&source=15
but check that they aren't too deep for the switch box.

However, I wonder whether a neon/LED will be prominent enough in
daylight, or too easy to ignore even if it is.

I was going to suggest a grid switch panel (single gang, 2 grid modules),
one 10AX switch and one grid indicator (amber, red or green). Those are all
designed for easy wiring and of course 240V.

The problem remains as to where to get a neutral from. I'm not sure what
current a grid neon takes (largely determined by the series resistor, but I
wouldn't advocate linking it to earth.

B&Q sell the MK range of grid stuff including indicators, but GET do a nice
range with pretty switches and nice panels if you need to match some
existing scheme eg brass or white nickle etc.

Cheers

Tim