I'd still suspect something bridging the switch. It would not need much.
Take the switch off then test it and see if it flashes.

I've seen this with these fangled electronic ballasts before.
Really irritating.
Brian

--
From the Sofa of Brian Gaff Reply address is active
"harryagain" wrote in message
...

"GB" wrote in message
...
Flashing bathroom light - whilst switched off!

This is 2 yrs after a complete rebuild. Recently, the bathroom light has
started to flash whilst switched off. It flashes very briefly once or
twice a second. It's a fluorescent 2D fitting, two wires, no earth.

I suspect one of three reasons:
a) Problem with the switch that allows a small amount of current through.
Seems unlikely. The switch looks fine, and the light switches on and off
okay.
b) Water ingress into the fitting. But this looks fine. No sign of water
damage. And, in any case, it's switched off, so it should have no live
feed. (Yes, I know that live and neutral sometimes get muddled up, but
even so where's current going to without an earth?)
c) Water ingress into the wiring somewhere where I can't see it. This is
in a flat, so $deity knows what the person in the flat upstairs lets
overflow.

Pull the fuse/turn the mcb off and see what happens.
Strong electric fields can cause a fluorescent tube to light up.
HV cables nearby. Source of micrwave energy.