George concealed flush cistern letting water by

[AP77]

Hi, trying to fix my daughter's toilet which has a constant trickle of water through the flush pipe into the pan.
The flush mechanism is Grohe 27.05.44 (see photos).
Reading other forum messages:
Is this most likely to be the seal at the bottom of the flush unit?
Are there any other possibilities?
Is the best option to just find a replacement? Thanks.

 

[polesapart]

Gone 27.05.44

Grohe?

Sometimes turning the seal upside down buys time while a replacement seal is sourced.

 

[AP77]

How do these actually work? Does the water just fill through the hole of the Grohe part and push the seal down? Are there any adjustments possible to this? I see various bits turn but can't see if it it is adjustable in any way. Thanks.