Could a Syphon were the handle is possibly not fitting correctly to the hook cause strain and cause the seal between the syphon and the cistern to leak.

If it’s not leaking when flushed, the it seems to me the syphon backnut, silicone where the tail is, or something obstructing the seal. If it’s any of these, the. Swapping out for a flush valve might not achieve anything.Seems ok at the start then leaks even when not flushed. Thinking of swapping for a flush valve and push button

Hard to tell from photo, could be ideal could be twyford couldn’t be generic. Is there a make on the toilet cistern?I think that's an ideal standard one
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