Follow up to earlier post. The actual model was the HNV170 Nextra.
I replaced the relay which I sourced for £5 all in but this was not the problem. The control PCB simply operates the relay to latch on the power i.e. the power switch closes a circuit to operate power to the relay, which has contacts in parrallel with the ON switch, so once the ON button is pressed, it operates the relay and the power is maintained, however, the power for the relay (24VDC) is derived from the bridge rectifier on the board (the little blsck round component), which smooths an AC voltage to DC to operate the relay (in conjunction with the Resistor Capacitor). The problem was the capacitor which is on the PCB to smotth the course DC voltage from the rectifier, hence with the capacitor faulty the voltage to the Relay operating coil was (from distant memory of this sort of thing) was a DC half wave sinusoidal voltage which needed the capacitor to produce a smooth DC voltage, hence the buzzing as the relay was continually operating. Hey presto, chnage the capacitor - the white tube looking device (£2 ish from Maplin) and it's fixed. I'd advise you to take the whole PCB to Maplin and get them to recommend the relacement device. Again, you will need a soldering Iron to replace the component - take care always as you are working with an electrical appliance (always switch the power off and take the plug out from the mains)
Tommy MacSuibhne, November 2007