Yep! Dave is right, the resistor pack is the cause.
Lift the bonnet, find the clip that holds the black panel just in front of the screen on the drivers side, remove it and lift the panel out.
You will see the heater fan housing. undo the two screws that hold the pollen filter cover, lift the cover off and pull out the paper filter .
This is a good time to clean or replace it if its really clagged up.
Next undo the screws that hold the small cover where the two wires enter and pull the resistor pack out. (It looks like three strips of metal and some coils of wire.)
Check for signs of burning and if it looks OK, clean it up carefully with a sharp pointed tool like a small screwdriver and a bit of emery cloth on a stick (G/F,s emery boards are good also)
You can check its all working before you re assemble it, but dont run it too long out of the airstream and keep your fingers clear.
If its broken, replacement is the only other option.
As Dave says, water ingress can be a cause but the other possible cause is a lack of airflow over it to cool it because the pollen filter is blocked, so check your filter at least twice a year. (it shold be done on a service anyway)
When you are finished, make sure the little black plastic rain channel goes in place and clean all the dirt and leaves that accumulate over the drain holes, before you refit the cover and fixing clip, this is important so as to prevent water getting in again.
