I've got about 18 radiators with Drayton TRV4 valves. I'm not sure
how old they are as I can't see any date stamps, but I suspect they're
about 20 years old.

I'm finding that many of the TRVs shut off at a fairly low temperature
(around 20 degrees) despite being turned up to MAX. Removing the
sensor head completely solves the problem, so I suspect that some of
the sensor heads are starting to malfunction due to age.

Is this a known issue with old TRV4 sensor heads and should I plan on
just replacing the problematic heads, or does this behaviour often
indicate a problem with the valve itself like the spring becoming too
weak which means I should be considering replacing the valves in the
summer?