Dimmed lights and extractor

is it possible to wire a timed extractor with the perm L and N coming from a independent supplied fused 3A spur, then connect the switched L from the lighting curcuit to the fan, tried it but it seemed to bypass timer function and make the fan run all the time, does the perm L and N have to come from same circuit as the switched Live? PLEASE HELP ME

P.S The bathroom lighting that i ran the switch cable from are controlled by a dimmer switch,

Why would you want the L/N from a different supply to the SL?This is dangerous if the FCU is on a totally different circuit to the lights!

The SL would not work if it is via a dimmer and the dimmer is turned down

I hope your dimmer is not in the bathroom!!

Unless in Z3 at a pinch...