Tool to undo the non return valves on shower mixer?

[ExteriorMonk]

I am trying to undo the non return valves on my shower to clean them but they won't budge, I guess there is a tool for doing this but can't find one on line - help would be appreciated!
It's basically a very wide slot but my largest screwdriver does not do the trick, the maintenance instructions say "use a suitable tool to prevent cross tracking" but don't say what the tool might be called.

 

[Lou]

Have you managed to find out what it is called?

 

[Chuck]

Without a photo that includes a scale it's difficult to be sure but I think that you may be looking for something called a "drag link socket".

 

[SimonG]

Big chisel often works for me, sometimes with an adjustable on it.