The fan is installed in my son's home, so I am not on the spot. Anyway, the fan is installed in the loft midway along the trunking. What could be the problem at only 2 years old? Could there be a sensor to check before replacing the whole unit?
Thank you for checking this out.

Simon Gest, September 2009

You might consider removing the fan unit temporarily and testing it on a bench. Then you'll know if a new one is required. Make a note of the connections when you take it out so that you can rewire correctly.

Vent fans often have Live and Neutral connections and a mains turn on trigger wire from the light switch (Switched Live) - when the fan "sees" the light go on it will run until the light is switched off and then run on for a while longer.

Peccavi, September 2009