Drayton TRV Where is the "give"

I have some Drayton TRV3 units which are probably 15 years old. I find that on the frost setting they do shut off the water supply and the radiator goes cold but only at room temperatures of 15C or so, at lower room temperatures the radiator is full on even on the frost setting. I assumed that the wax capsule (or whatever is in them) had broken and ordered some replacement heads. (which being TRV3 to TRV4 converters are not as cheap as simple TRV4 heads). Anyway having fitted the replacements curiosity got the better of me and I took one of the old ones apart.

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found that when taken from the cold outdoors to the warm indoors the extension of the actuator pin in the TRV head (ie the bit that presses on the little pip in the water valve) varied between 20mm and 25 mm,

(Dimension A on here)

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which indicates that the wax capsule is indeed doing its stuff. Furthermore the very end of the actuator arm was a screwed in brass hexagon, and if I was a betting man I would say that when manufactured there is/was a calibration table to show the total length of the actuator arm for the ambient condition in the factory.

My thought process is if I extend the length of the actuator by a turn or so this should shut off the water supply at a lower temperature driving the room down to say 10C (or lower) on the frost setting.

My concern is when the actuator extends to maximum length and the water valve is closed where is the extra movement of the actuator absorbed? If I press with my thumb on the actuator shaft (the same way that the pin in the valve will do) there appears to be no give or resilience.

On a completely un-tampered with unit this would be the equivalent of what happens to the unit when the head selected temp is "frost setting" but the body temperature reaches say 30C in high summer. The valve was closed at 5C but now its closed and a further 25C of expansion needs to take place - where is the spring give?

Any ideas or help would be appreciated as this might mean that I can fix at least some of the remaining 4 heads that I have so far not bought replacements for.

Chris.