Enclosed domestic cistern help please

[Invicta]

We have recently moved into a house that has had a downstairs loo put in.

the ball cock working with the Armitage Shanks loo isn't fully closing so there is a constant trickle of water going into the pan.

As we are on a water meter I would like to fix/replace the unit so the water flow stops running properly.

Unfortunately the cistern is boxed into the wall by a couple of (mdf?) panels.
The top one has the handle mechanism in it which consists of a round plastic/chrome ring (approx. 7.5 cm Dia) in it on the right hand side in which there are two segments, the smaller one gives a short flush and the larger a longer flush.

I have tried to move the top panel up to get it off it's fittings but the round handle unit prevents me.
I cant move it down or sideways as there is no gap to allow this.
How would this round handle unit be fitted?
Do I have to unscrew it anticlockwise, prize it off or do something else.

I can't fully lift the panel up as a top "shelf" has been fitted and grouted into the surrounding tiles. This has a lip which stops the panels from sliding upwards.

Any suggestions will be appreciated.
Thanks for looking,
Bern

 

[gassafeengineer]

Turn the other ring anti clockwise to unscrew it.. Try this first

 

[Invicta]

Gassa,

Thank you for this.

I was worried that if I tried to unscrew this I might break those two push activated segments of the flush mechanism.

I will give it a go,

Bern

UPDATE
The panel hinged forward (like a cupboard door on it's side).
Behind it I found the pipe which joins the cistern to the bowl was leaking at the join with the cistern.
I've turned the water supply off via an inline valve.
I will start a new question as to what best to do next.

Your help is VERY appreciated as it looks like this has been leaking for some time.