Karcher K2 pressure won't change

[Richard Livingstone]

I've had this pressure washer a long time and it's occasionally used. For about two years (I've not done anything about it) the lever that changes the pressure seems not to have any effect. I can't find anything more about this online and I just dismantled the washer to see if there was something obvious but nothing really, the lever is attached to the motor and moves a screw when lever is moved,(see picture attached) but doesn't do what it should. Water pressure is ok, no gunk in inlet and outlet that I can see. Anyone got any ideas other than buying a new one?