Unable to remove shower faceplate retaining rings

[bigglesuk]

I need to fix our leaking shower but can't remove the faceplate retaining ring. It just keeps spinning around and around no matter which way I turn it. I've tried putting a bit of pressure pulling away from the faceplate when unscrewing it, but nothing. It feels as though whatever it screws into is loose behind so both the ring and what it's attached to are both spinning round.

Any ideas on how to remove it?

 

[blup]

The thread may have become ineffective because of a build up of limescale, or rust.

Blup

 

[bigglesuk]

The thread may have become ineffective because of a build up of limescale, or rust.

Blup

I would have thought that it wouldn't have moved at all if there was a build up of limescale or rust?