Hudson Reed shower - need help with identifying the solution to the water dripping problem

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I had this exposed valve thermostatic shower system installed about 2 years ago. It has 2 shower heads - a round overhead mount and handheld attachment. There are 4 valves in total: one to select a shower head, one to control the water flow to the shower heads, one to control the water temperature, and one to control the water flow to the bathtub spout. The system has now developed an annoying, constant water drip from either shower head, depending on which one is selected with the shower selection valve. From watching various youtube videos and reading online, it seemed they suggest replacing the thermostatic cartridge as the solution. However, with the shower flow control valve sitting separately above the thermostatic valve (see the attached picture), I'm not sure if replacing the cartridge is the right thing to do here. Is the cause of the drip a faulty flow valve rather than the cartridge? And if yes, is there a link / reference material that explains how to get that fixed or replaced?

Thank you!