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Light_EDDed-
There are lots of different approaches, levering or tapping the loose ends apart. Maybe a box spanner would work on that nut.
As others have said though, good luck finding a ceramic cartridge that fits. I know some guys carry a set of them and there's a plumbers merchant near me that has a lot of different ones. Occasionally I've had some success but usually with shower mixers not taps.
Airhead
Guess you need a deep thin wall socket. Of course remove the circlip as @bluehuw has already mentioned.