Yep, the mosfet is definately not switching properly. Having looked at it again I actually only get 3 states at this frequency: off, on and 30% which is when the mosfet heats up and i guess its neither on or off.
The cap didn't make a difference. It just seems that the mosfet's gate is not charged/discharged quickly enough. The mosfet driver will tell. At least the humming is gone =).
Cheers,
Adrian