Not sure...

[Dave]

Can you hear your pumps pressure releif valve kicking in when you try and spin it? 'cos heavy steering is normally the noticeable flat tyres, or problems with your PAS pump, whats it like compared to when the car engines off, does it change how the steering feels. Pick a quiet road and switch your engine off and coast to see if it changes, remember dont remove your key though! else your lock will engage