Wasn't trivial.
New firevalve was dismantled and had lots more O-rings, so tried to fit the whole top assy. Wouldn't tighten! wrong thread, so ended up using the actual main barrel housing (in situ) plus the old shaft bearing fitted with the new plunger/spring and O-rings and a new O-ring on the shaft seal plus a new thumbwheel.
This mixture of old and new fitted and sealed.
And doesn't bloody well leak, either. It may not actually cut the oil off either but I am too irritated to care
Since I have different ways to do that anyway..
Aga now lit..we shall see.