A clogged oil screen is possible, but if it has that much crud in it, the pump gears might be shot, too. Since you'd have to access the pump to clean the screen, you might as well put a new one in. Another possible cause for low pressure is worn-out main bearings. The mains shouldn't be gone at this mileage unless the engine was run with dirty oil, or none. I have experienced one instance of low oil pressure resulting from a clean-out plug being blown out of a crankshaft, but this was not on a Rover engine. This is highly unlikely.
Good luck!