Toilet issue

[Steve]

My toilet, at the ripe young age of 6 years old, has started letting water through the cistern. It only happens on some flushes. Doesn't matter if its a half flush or full flush. It's an ideal standard toilet. I've attached pictures of it's innards.

So here are the two flush modes.
1. Push the button. Water empties and the syphon unit clunks as it closes off. No further water passes.
2. Push the button. Water empties and the syphon doesn't clunk as it closes. Water passes through slowly continuously until the next flush.

Is this repairable somehow? Or will I have to replace the syphon unit? I've replaced an old fashioned flapper syphon before but would rather not take the toilet apart if I don't need to! Somethings telling me this syphon should twist off the base but the only bit i can see that might twist off means twisting it the wrong way - it won't turn any further that way.

Thanks.

 

[HERTS P&D]

This is the part you need:

https://www.ebay.co.uk/itm/152814835572?mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5338785333&toolid=10029&_trkparms=ispr=1&amdata=enc:1Bk3ILBXvRHGr1Aa8aFhyWg21&customid=s:GS;gc:EAIaIQobChMI3aPi6aDk9QIVDurtCh28owo_EAQYAiABEgI4UfD_BwE

But looking at your picture, you would need to turn the flush valve anti-clockwise to remove, but your flush valve is already touching the back of the cistern.

So it might ne a cistern off job to adjust the flush valve.

Andy