| Gulp. I'll get an electrician!
No, you get (assuming it's a timer over run fan) a length of
triple-and-earth cable and a triple-pole fan isolator switch (which are
readily available in the sheds). (Switch off at the mains first, usual
Take the cable from the light to the isolator switch location and then from
the switch to the fan. The switch needs to be out of reach of the bath or
shower - google Bathroom Zones for more info.
At the light, terminate red on the Permanent Live (this will probably
already have 3 reds on it, 2 for the lighting loop and 1 for the switch
feed). Terminate yellow on the Switched Live (this may be a single red for
the switch return, or a black with a red sleeve, or just a black, but will
connect to one side of the bulb). Treminate the blue on the Neutral (this
will probably already have 2 blacks on it for the lighting loop, and connect
to the other side of the bulb).
At the switch, just match up the red yellow blue on the supply side of the
switch to red yellow blue on the load side.
At the fan, terminate red on L, blue on N, and yellow on the teminal
probably marked Trigger.
All earths need green/yellow sleeving and to be connected up even if the fan
is double insulated.