I guess you mean a "Timer" fan, Derek? A regular fan, but having an adjustable overrun.
Your fan probably has three wires (switched live/neutral/earth.) You'll need a fourth (permanent live.)
How easy it is to bring in the permanent live depends entirely on how your lighting circuit is set up.
Assuming your fan comes on with the light switch, the usual thing is to run a wire from the ceiling light back to the fan. Or, better still, remove the existing "twin & earth" cable and replace it with a "three core & earth"