Morning all. I’m hoping someone can help. My very old bristan mixer shower has been dripping so having done some research I decided to take it apart to try and maybe soak the cartridge in vinegar and put new O rings in (we live in a very hard water area). Problem is I cannot get the outside casing off and I assume it’s because the white handle (see photo) needs to come off first. However I cannot get the handle to budge. Am I doing the right thing? Tia.