What sort of transformer for the LV lights? Electronic or normal? If
electronic, changing to a wound one one should do it (but make sure it's
suitable). Or add a relay. The fan voltage sensor must have a high input
impedance so is sensing at near zero current.


transformers are electronic, but the fan isn't on a transformer, it's 240v
the switch isn't suitable for wound transformers
just bought a replacement from TLC, as suggested by Sparks, but it's the
exact same switch just in a different box

back to the drawing board...