This is the thermostatic valve from a shower:
When the unit warms up, the section on the left moves outwards, opening the gap you can see to the right of the first black rubber seal. This allows more cold water to flow in through that gap, and out into the mixer.
The problem is that the mixed water never gets hot enough, so either the thermostat mechanism itself is too cautious about hot water, or the valve is unable to restrict the entry of cold water properly (I suspect this).
I think it's probably unlikely I will find a replacement for this, and instead will need to replace the whole mixer. Before I do, any suggestions for identifying it in order to find a replacement?
Thanks,
Daniele