Mixer tap cartridge replacement help

[Westendboy]

First post here.

I'm trying to replace the cartridge on our bathroom mixer tap as it has become very stiff. I've taken the tap head off and unscrewed the shroud with no problems but I can't remove the metal bit that the shroud screws into to enable me to lift the cartridge out. All of the youtube videos that I watched show that this should be a nut and is undone with a wrench/spanner but mine doesn't seem to have a nut and is just a thread so there is nothing to grip onto to undo it (see photos below). Any ideas?

 

[dilalio]

A picture of the replacement cartridge would help but either the slot in the brass thread could be used to apply torque for removal or the cartridge just pulls out as is.