Toilet flush tank leaking into bowl - unable to figure out mechanism
It's a pneumatic valve

https://www.wolseley.co.uk/product/sv93467-pneumatic-dual-flow-valve-15-pc93209-44/


according to this page:
https://plumbsparesdirect.com/store/sv93467-dual-flush-valve


  If the flush valve is letting by water into the pan the most likely problem is that the flush valve washer has failed
  To replace the Ideal Standard flush seal and clip turn off the water and empty the cistern then simply twist the body of the flush valve from the cradle, pull off the worn washer push on the new one twist the body back into the cradle and then refill the cistern
  Please refer to attached diagram for further fitting instructions