here is the situation.
Loop wire into Junction box, Switch wire out to switch and then wire to single GU10 Lamp (with an LED 4.5w bulb)
Wired as as single switch it all works fine. No problems.

If I change to be wired as a 2 way switch (including correctly wiring up the 2 way switch) every 30 seconds or so my LED Lamp does a tiny flash in the OFF position!.

So, I took a new piece of 3 core wire and used that to connect the 2nd switch to eliminate any issue with the installed wire. Exactly the same effect.

So, I tried the following.

connect main live and Switched live to L1 and L2 in master switch. (no flashing but obviously can't tun the light on).
connect Common wire only in new piece of 3 core to master switch only, leaving wire "flying", (no flashing but obviously can't tun the light on).
connect from 3 core to L2 in master switch only, still leaving flying. – Starts flashing again!

is this some sort of induced voltage in the 3 core cable? even though the ends aren't connected to anything? it really makes no sense. (well not to me anyway).

thanks in advance for any pointers.

Are you defo switching the phase not neutral?

Well, the Junction box (the non service type) is now buried in the ceiling, I probably could get to it by lifting a floor board. I've put plenty of other JBs in for the lighting and not made that mistake.

If i was making that mistake wouldn't you expect same issue just wiring up as a normal single switch?

I've had some weird happenings in a house that had a broken neutral wire in a junction box mainly a mains smoke alarm that would make odd noises & a shaver point that had a pulsing flicker to its 'on' light – both disappeared when I fixed the break.

thanks snaps. if it were a junction box with a neutral issue, i would expect the issue to be apparent when wired as a normal single switch. I might have to pull up the floor board to check the JB anyway though. damn it.