How to remove recessed cartridge from bathroom tap

[pinkmonkey]

Hi,

Dripping tap, thought I would have a go at fixing this myself.
I've removed the handle & sleeve but can't figure out how to remove the cartridge?
I can twist the white square spindle 1/2 turn only & it won't pull up on it's own?
I've attached pics. Thanks

 

[avonmouth]

Does that hexagon nut shape unscrew ?

 

[Wayners]

Have to tried to undo this

 

[pinkmonkey]

Thanks! I thought that was part of the tap itself as so narrow! Sprayed with WD40 & managed to get a grip and off it came!
Next question, I see these are readibly available on Amazon but can I source at plumbers merchant/screwfix/b&q?

 

[pinkmonkey]

Ah, found on screwfix! Thanks all