I've changed ours a couple of times.
There will be a grub screw that holds the cap with the lever on. Once that is off you will be able to see the cartridge but probably not get it out. Another layer of the silver trim will probably unscrew (that castellated bit) and then you'll be able to get a large socket onto the cartridge and remove it. Having the cartridge in your hand doesn't mean you'll be able to identify it but a plumbers merchant may. Don't forget to switch the water off first!