Leaking shower remedy

I have a ten year old Provost Riser Rail System with Integrated Shower Mixer and Thermostatic ABS (Ultra) (pic attached) which is now beginning to leak a little around the on/off tap on the left hand side. It might just be a washer that needs replacing, but there is no obvious way to gain access to the cartridge mechanism inside the bar. Is there a way to inspect the washers or will I have get either a new compatible cartridge (if there is one) or a complete new bar assembly?

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