Removal of retaining ring on thermostatic shower valve

[cheradenine]

Our thermostatic shower valve is misbehaving (1 year old, bathstore, not impressed).

The instructions casually say "remove retaining ring" (see attachment). Um... how? It's threaded and pretty stiff; tools can't really get to it - forcing a pair of screwdrivers in gives no joy. I even tried fabricating something out of a bit of pipe, but the lugs just sheared off.

Is there some 'magick tool'? I perused toolstation and couldn't see anything..

 

[Madrab]

Get a chunky plain screwdriver, punch or narrow cold chisel into the notch, use a hammer and 'chap' it round anticlockwise.

 

[cheradenine]

Ta. That worked. I was a bit nervous of damaging it, but it wasn't actually that tight in the end.

I shall pay much more attention to the serviceability of the next one I buy!