Neon in isolator switch only lights when live is conected?

This is for a shower, it doesn't seitch on...

I looked at the swtich and no light is on. I took it off the ceiling and as soon as I disconnect the neutral the neon comes on; I used a neon screwdriver and it lights on the live, but when I put a meter on the wires, no voltage shows.

If I connect just live to the switch, the neon will light; as soon as I connect neutral it goes out. It does flick on very very briefly when the switch is switched on, like there is voltage there for a brief second, but not enough to power anything.

I think the RCD is defective, but it does trip when test is pressed - I can't light a mains light bulb from the live side either, but it does produce a very tiny spark.

Any ideas?