I decided to reroute the shower fan off the main light switch so I can run it independently in the summer. There are two light switches, one for spots, one for the overhead light. I wired up the new fan unit behind a pull cord and a isolator switch outside & above the bathroom door. Today I connected the new circuit into the overhead light circuit. All good until I turned the circuit on via the new isolator - the spot lights starting flashing. I never touched that circuit so that's very strange. It's hardly induction from the AC by wires crisscrossing in the attic is it? I'll have to trace the spot light to switch cable and see where it feeds off but was a bit surprised! I reverted until I investigate but any ideas why this can happen - is my new wiring that bad?!! Thanks. 

Just to be clear, are the spotlights flashing when OFF and on normally when on?

 

And are they LED lights?

They are LED lights and operate normally on their own switch. But when they are left switched off, and I disable the isolation on the new shower fan circuit that's when they start flashing....!! 

First things first, for safety.

Check your earths.

So when the fan switch is ON the lights flash, but when the fan isolator is OFF the lights behave normally?

 

You are getting leakage from the switched L of the timer fan circuit.  If you have fan and light on a separate switch I would disconnect the timer, it's not needed.

After the earths are checked, fit a capacitor across the lamp terminals that will dissipate the charge that’s causing the flickering

Thanks - think I'll trace the switched live and see if I connected that through ok - was hoping it would run the fan based on the humidity sensor even after the pull cord was turned off. Then I'll trace the spot light cabling and make sure it's not spliced anywhere it shouldn't be. Cheers. 

+1 to what @ProDave and @TonyTare saying. 

 

As a check you can temporarily connect an incandescent bulb/pendant light in parallel with the LED if you have one. If that stops the flashing then add a class x capacitor in parallel. Any value from 0.047uF to 0.47uF usually works. Put a note about the capacitor in the CU.