Mixer Tap Cartridge Removal - It wont come out!

[bfevo]

Hi

I have a Wickes bathroom mixer tap and the cartridge has gone, i think its what they call a style mixer as it goes left and right (hot & cold) and up and down (on or off).

I have take the head of the tap of and exposed the top of the mixer however the collar that surrounds the mixer has no indents for a wrench its perfectly round, the cartridge itself seems to have locating holes for a removal tool!? How the hell does this thing come out............... Please help me not waste £40 on a plumber!

Collar with no indents:

Top of cartridge:

 

[Burnerman]

This could be one of those jobs where it could be a good move to have the replacement cartridge standing by......however one technique is to wrap a good few turns of masking tape around the round boss, and then trying to undo it with some slip joint pliers.
There doesn't look to be a big deal if the boss gets slightly marked anyway?
John

 

[JimCrow]

A box spanner will fit the notches on the inner ring,

 

[spareshunter]

Please help me not waste £40 on a plumber!

Waste

Free advice; get new tap more trouble than worth.