As Mr Sworld says it'll be the mixer valve. I sadly could find nowhere that had the same one as mine, there appear to be 1000s and bought a whole new part like Spindrift did, but fitted it myself without too much trouble
Have found a merchant who says they have one type of valve in stock. Screwfix next door so will get a whole new assembly if not compatible.
@lmamammal started
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As Mr Sworld says it'll be the mixer valve. I sadly could find nowhere that had the same one as mine, there appear to be 1000s and bought a whole new part like Spindrift did, but fitted it myself without too much trouble