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Felton Shower Mixing Valve problem

My hot water cylinder was a low pressure, now replaced by an outdoor one(seems high pressure). I have two showers, the one close to it when switches on, has water leaking from the connection to rotor. I tried to tighten the shower hose , leaking still on, and I damaged the valve control sleeve and grub screw. My question: 1: Do I have to change the mixing valve to high pressure and how much ? 2: Maybe the grub screw area got damaged , need to replace one as per pictured. I need a quote, thanks