Continually running Roca Dual Flush

[cerclebois]

We have one from 2008. This morning it started running non stop after it was flushed, and the flush button gives no resistance to pressure.

I'd like to see if I can cure the problem but can see no way to remove the cistern lid. I think it must be via the flush fitting. How do I remove the lid, please--tools needed?
What is likely cause of the malfunction, and its solution?

 

[Aquaheat]

Flush button will unscrew anti clockwise to release the cistern lid.

Jon

 

[Aquaheat]

Most likely siphon stuck in the open position, siphon washer has failed completely or siphon washer has split/perished.

Jon

 

[cerclebois]

thanks all. By chance my son-i-l popped round and dealt with problem very quickly. Limescale!