Heater Valve Background
The only function of the heat sensor tube is to close the tap when exiting air gets too hot, using expansion of the fluid inside it, which is heated by the exiting air. So as the valve is permanently shut, even when cold, it is not that.
The SM heater has over closed capability. This means that as you go past the start of the white section towards the blue the valve is already closed, but the actuator, being mechanical, continues to travel in the same direction.
What is wrong with this one is that the tap closing diaphragm is stuck up on the sealing face. The pull off mechanism has broken. I surmise that this is caused by the diaphragm gluing itself to the sealing face, then pulling off the pin that retracts it. As the sealing spring is quite strong, once failed, the pull off pin will not work again.
Recommendation: Store your SM with the heater valve open.
Other than removing and rebuilding, there are two "solutions" that defeat the temperature sensor spring, and thus revert the control to manual (no automatic thermostatic adjustment)
Lay on your back with a flashlight (torch) and determine if the valve is responding to the lever and linkage. If it is it is probably a faulty heat sensor tube (I think they are called borden tubes or something like that).
Cut the spring
If you reach into the heater valve spring cage with a very good pair of side cutters or nippers and cut the spring in one or two places the valve will revert to a manual heat control. In other words, collapse the spring. The spring material is extremely hard.
(or) Add a Spacer
Put a short slug made from a 5mm bolt into the head of the heater capsule and all is well. But you do have to take it apart, nearly a 5 minute job (really).