I went into town to get an o-ring. Just an o-ring.
Our new to us MF 65 tractor arrived with a hydraulic diverter valve that sent oil splurting out over the gearbox if you bumped the lever but, worst of all, the 3 point linkage out the back wouldn’t move. Removing the valve off the tractor and bopping out the diverter piston showed that the o-ring had failed.
The o-ring shop also sold chainsaws.
So we now have a new chainsaw, a fixed valve and the 3 point linkage works! Wins all round and it's only just gone 10 am in the morning!