Drayton TRV4 Thermostatic Radiator Valve Problems

I have a radiator with a Drayton TRV4 Thermostatic Radiator Valve which had been working perfectly for years. However, the radiator does not completely go cold anymore, in fact is still quite warm, when the heating is on in the rest of the house.
I have removed the thermostat head and examined the pin in the valve below. I can only get a very small movement in the pin even when I have given it a few lightish taps with a hammer. This amount of movement does not seem to be anywhere near as much movement as the plunger in the thermostat head when I wind the control fully in and out on the TRV head.
My question is:
Should there be more movement in the pin on the valve than I am getting ?
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