No make sure power is off else you will blow a fuse (if your have a UK plug the fuse will blow in the power cable else in the box itself). I did say earlier that flicking the switch back and forwards did the trick which it did but after a while it stopped working again - cheap components I suspect. I was still under warranty so called Dell and they said you have 2 options:-
1. Send box back to them and they will replace free of charge
2. Pay £80 (plus VAT) for an extended 2 year warranty taking it to 3 years all told. They then sent around an engineer to replace the power buton and all OK now.
The bottom line is that I could have sent in my PC to Dell for repair (via courier could be expensive ) but then you are not sure about your data, personal files etc. I opted for the £80 option and all ok as the repair is done on site at your house. The engineer was excellent and I am sure those who are outside their warranty period could find someone locally for a cheaper price than £80 but then you don't have cover if it happens again. My way I am out of pocket £80 but have peace of mind for another 2+ years.