Rover Streetwise power steering very light?

I've recently bought a Rover Streetwise 2.0 TD SE which although 5 year old has only 25000 on the clock. I've found that the power steering is very light to the extent that if I go over any slight bump. pothole etc it veers me all over the road. Having had Rover 200s in the past I've never encountered anything like this - is this normal. I've had the steering etc. checked out and everything appears ok