Surely if they are not turning off with the decorator cap it implies the
rubber washer has disintigrated.


No, not at all. If the pin is stuck (which most of mine have at some
point) then no amount of reasonable force seems to free it. But just
relaesing the inner nut half a turn always does free it - don't know
why. Having freed the pin and applied a bit of WD40 never had the
problem again on the same valve. just giving you my experience with
Peglar TRV's.


I would have thought the decorators cap - to screw down, must be depressing
the plunger all the way as that is its role. If the plunger is right down
then why wouldn't it turn off?