Force 10 shower

My son is having problems with the shower in the house he has just moved into. The shower is a "Force 10 Thermostatic" with a single lever to control on/off and temperature, and is "Victorian" style (i.e. brass and porcelain).
The problem is that the water temperature is virtually uncontrollable and too hot for comfort. The water is heated by a combi boiler.
I have dismantled the valve and checked the thermostat capsule in a bowl of hot water and it appears to operate OK.
I do not know if "Force 10" is the manufacturers name or a model/type name. I cannot find any references to this name on the internet.
Has anybody come across this name before? Any ideas what to do next to correct the problem?
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