Hot water doesn't turn off on shower

[BCarron]

I have recently moved into a property which has a thermostatic mixer shower. Sometimes but not always the hot water continues to flow after the shower has been switched off. When this happens it can sometimes be rectified by putting shower head on floor waiting a minute and switching on then off, otherwise I have to stop the flow from the water tank and even then the shower tends to flow again once the flow has been restored.
I am guessing that there is a pressure controlled valve for the hot water which is sticking some times. I have looked in the loft and there does not appear to be a pump, also there is no make or model indicated on the shower, though it is a flush fitted chrome boss type.

Is this repairable or should I replace it?