I've got a central heating system full of 30 year old TRV's.

Now and again I need to tap the pin to free off a valve, and I'm
unsure whether some of the valves work correctly at all.

Is there any simple means of testing the valve or just the actuator?

If I decide to replace, does anybody have a particular
recommendation?

I assume that the idea is that the valve senses the room air
temperature and controls the water flow, and this can only be
approximate if it's actually attached to the rad.

Thanks