IanK Spec C

Since yesterday evening my steering appears to have gone overly light as if the power steering is no longer speed sensitive. My tyre pressures are all correct, I've not hit any pot holes or kerbs etc. The power steering fluid level is correct. Car corners fine, it just doesn't feel right. Power steering works fine a parking speeds but just doesn't seem to firm up as the vehicle speed increases. Car's has covered 18,000 miles. An ideas as to what's gone wrong ?

Ian