Have you checked the rack bellows?(Rubber boots at the end of side of the rack).They could have let grit and dirt get in to the rack and ruin it.That could explain the mild grinding.but then again if the MOT man didnt find anything wrong with them, then its just worth checking to make sure he didnt miss something.Is it possible that the engine was ran with no fluid in the pump?(This could of happened before you bought it).If so this ruins the pump.Or worse case scenario,the rack or pinon could be worn.I got this info from my Autodata manual and included some of my own thoughts.