Fan won't stop running on Ideal mini S28?

Hi,

I've got an Ideal mini S28 boiler which is working fine, except that when the CH/HW program is off, the flue fan sometimes keeps running for hours. For example, the HW went off at 8:30 this morning, it's now 1:00pm and the fan's still whirring away.

If I turn the dial on the boiler itself off (i.e. to the O position), the fan stops, but as soon as I turn the dial back, the fan kicks in again.

Does anyone know how I can fix this?

Thanks,

Neil