Replacing diaphragm in cistern

Technoi

Hi All,
The toilet has been running into the bowl, had a look and saw water leaking out through the top of the fitting pictured. Now I figure the diaphragm needs replacing. Problem is I can't seem to open either of the plastic nuts?.
I can't figure out if the fitting is disassembled from the bottom up or from the top. Either way I can't seem to twist open either of the 2 nut things nor have I room for clear purchase on them.
Any ideas? Has it been cemented closed? or is it the norm to replace the entire unit.
Any ideas of how to open the fitting would be much appreciated.
Granted I'm not overly familiar with plumbing but would like to fix this myself if possible.
Thanks in advance!

sullzz

Thats an armitage shanks brand ballvalve , you would be better off replacing the whole thing if you follow the silver flexi pipe back you will find a white quarter turn isolation valve which will isolate the water supply , disconnect the flexi to the ballvalve and bring it to smallmans in ranelagh , on sallymount avenue , if you show it to them they will give you a replacement one , i got them there before.

Technoi

Brilliant Sullzz. Many thanks for the description of what to remove and the shop details. Hopefully have it replaced by the weekend.
Thanks again!

Technoi