Could be a number of things. I believe that diesel has 'drive by wire' which means that the accelerator pedal isn't connected to the injection pump (engine) via a normal throttle cable. It is in fact connected direct to the Engine Management Unit (system) using a sort of potentiometer sending data signals. It could be the actual pedal sender or the actual unit. Of course it could be something entirely different but I think you will have to take it to a Rover garage and have it diagnosed via 'TextBook' if they still use that system.