Replace mixer cartridge tool question

Hello,

Another 'what tool do I need' question!

This is for a mixer cartridge on a shower, there appears to be a collar, preventing the cartridge pulling out. The slots on the collar are recessed.

I've tired open pliers at right angles but can't get the torque without slipping.
I've tried adjustable grips on the cartridge.

I wondered if a hook spanner may work, but as it's recessed I'm not sure how.

I'm guessing a specific tool?

Any ideas?

Thank you in advance.

 

Not worth messing about with, just buy a new shower bar.

Andy

 

Any recommendations for makes? The mixer on this one went after about 18 months.

It needs a dual outlet top and bottom, for fixed riser and detachable shower head.

 

https://www.simplybathrooms.co.uk/product/sh02-round-hp-dual-outlet-thermostatic-mixer-shower/

Comes with a 3 year guarantee.

Andy