This is a reply to Bruce:
“..Maybe the tach is wrong..” ..yup, maybe. Have you checked its connection to the side of the engine? Tried wiggling it to see if it’s loose? If the hours meter has gone wrong, then maybe it IS the tach which isn’t reading correctly. There are hand-held techs which respond to a bit of reflective tape which you stick to the flywheel – or a pulley on the front of the engine (Maplin used to sell them). That’d be a way to check whether the tach is correct or not.