I think using these has come up a couple of times recently. My experience in case it's useful to anyone. Apologies to anyone for whom this is the bleedin obvious.
MK state that these should not be used with fluorescents. (and should be wired *across* the switch contacts) I assume so that the trickle current through the neons also passes through the lamp with insufficient current to cause the lamp to light, but when the switch in on/lamp is lit the neons are shorted out)
Just fitted one to a fluorescent - and by design I have neutrals in my switch backboxes. Wired to the second contact in a changeover switch, and to neutral. Gives exactly the same action as MK recommended usage - neons on when lamp is off, and v.v.