pscan.uk

Error $22 is defined in the protocol standards as "prerequisite conditions are not met". It basically means that pscan is asking the ECU to reset its error codes but the ECU is refusing for some reason.
The exact logic of why the ECU is refusing is only known the MG Rover engineers who wrote the code inside the ECU.
My guess is that the ECU is still seeing an actual permanent fault with a sensor.
Another reason could be that the engine is running when you erase the errors.