Thanks for the info, guys. It turned out to be the belt: although it was tight it wasn't tight enough and well beyond its' sell by date! When taken off it was through to the canvas on the underneath. New belt fitted and now right as rain.
The power steering on my Outback has become lumpy when manoeuvering at low speed - parking in tight spot etc.
First started in winter, put it down to cold fluid, but now does it all the time. Fluid level is OK, belt is tight.