Hi, I'm thinking I need to change the cartridge in the bathroom mixer tap. It's very stiff and only runs hot water. I've located the grub screw (I think) but can't make out what sort of key it needs. It appears to be circular as a opposed to e.g. hexagonal. I've tried a couple of allen keys with no luck. Is it possible that it requires a specialist tool of some kind? It's a vitra mixer/lever tap. Thanks