Hi. I have Charlton Mixer Tap in my shower i.e. thermostat and water control. I need to change the cartridge in the tap as it is dripping badly. However, I'm really struggling to access the cartridge. The way to remove the tap appears to be to unscrew the chrome covering i.e. this will bring the actual tap 'turner' off with it (this is what I have to do with my matching Charlton mixer tap on the bathroom basin). However, I cannot make the unit turn no matter what. It seems to be seized up. There is no screw or grommet holding it place. Further, the surface is completely smooth chrome meaning it's really hard to find a tool that will grip it. I've used industrial rubber gloves that allow me to get a good purchase on it, but still I can't shift it. Any thoughts? I hope this makes sense. I've attached a photo with an arrow pointing to the collar/cap that needs to be unscrewed to bring the front off and expose the area of the cartridge that neds to be replaced. Thanks!