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.

 

[ShaunCorbs]

It’s one of them might be worth trying an impact screwdriver (not a battery one a hammer one)

 

[ExteriorMonk]

It’s one of them might be worth trying an impact screwdriver (not a battery one a hammer one)

I was thinking there might be a specific tool with a really wide slotted head, or a couple of spaced tabs?

 

[ShaunCorbs]

Home made with a mig welder flat bar and a socket

 

[exedon2]

I've a screwdriver that's intended to remove aga/rayburn top plate studs
Got a hex on shaft for a spanner
Works very well on showers as above.