This KUB3023 has what seems to be a Valquest cartridge held in place by a 52-53 mm ring nut with 4 relatively shallow slots cut in the rim. The nut protrudes around 3 mm above the mixer casing and the mixer is recessed into the wall. No joy with an adjustable wrench. I made a removal tool with lugs to engage in the slots from 3mm steel. Again no joy. Hudson Reed tech support useless- "depends on water hardness". What?? "Try a screwdriver in one slot or one on each side" !!!. I was told that there was no specific removal tool for the nut- a screwdriver usually worked (I would like to see this demonstrated). Repeated soakings with WD40, which can normally perform minor miracles, has not helped. The shower is only three years old. Has anyone had any experience with this 'easy to replace' cartridge/ mixer?