Neil, whilst stationary / idling try turning the steering on full lock each way if you hear the noise it is more than likely the power steering belt thats slipping.
Otherwise, turn everything electrical on and see if the noise reappears which would suggest a loose alternator belt.
If its a slight squeak all the time it is idling probably sh*te/rust on the belt pulleys ..... skoosh some WD40 on ther rear of the power steering / alternator belts whilst its idling. The noise should disappear.