double light switch?

Disconnect the cable for the fan from the light fitting, and take that to the fan switch.
Replace the 3 core you've just taken out with a 2 core cable, connecting brown and yellow in the same terminal, and the 2 blue wires together, and take that to the switch.

You may be able to do all that in the loft, depending on where the cables go and where you need the switch.

If you want the overun to work don't use a double pole switch.

Adam
February 2010