No need to change the entire brass fitting Spruce!I simply changed the plastic plunger,spring,screw thread and k *n*o*b and my heater is now on full heat setting.i did try to undo the nuts and they are on tight so left them.
I did what you suggested Splash and dash and just undid the threaded bit a little so a small bit is showing then screwed the k*n*o*b back on.i was getting 58c after 20 mins of firing up the heater(coiled hose temperature).too hot to leave my hand under(ambient air temperature 6c this morning).
Changed antifreeze in header tank(the wet and dry vac got rid of the old stuff),taped up a few of the wires that needed it then washed missus car.
So good to have piping hot water again ready for the new year!🙂👍
It's a pity you can't just buy the innards of the mixer valve rather than the whole fitting!works like a dream again now!