wiringnan extractor fan

Hello, I am replacing an extractor fan for my bathroom, the existing one is located in the loft with a 2 core electric cable connecting the fan.  The cable come from a switch next to the fan in the loft. My new fan needs a 3 core supply as its a timed fan. My question is can I run 3 core cable from the switch in the loft to the fan or do I need to replace the existing wiring (2 core) from the main switch located outside of the bathroom.  Thanks

It depends if you want to use the timer function.

How does it operate at the moment?  just comes on with the light or on it's own switch somewhere?

You also need to double check the physical location in relation to Bath and/or Shower enclosure..

There are certain locations, (called zones), in bath or shower rooms..

where some categories of mains (230v) supplied equipment should not be fitted...

If a 230v fan was incorrectly fitted in the wrong zone then it should be replaced with a 12v fan..

in which case your 2-core cable possibly could be used for a 12v supply from a separate transformer/timer unit.    

But some of these types of questions can be difficult to answer over the internet, without a site visit.

Timer fans off the light can be great when taking a shower..

BUT a pain in the rear if just popping to the loo for a pee!!!