I found some bubbles had formed on the the rubber ring that acts as a seal on the bottom of the cistern.
The whole cistern unit just unclipped from the cistern so I took the whole unit to a Plumb Center and they said its a common fault with Geberit and the ring needs to be replaced...
I have exactly the same problem as someplumb, did you manage to fix it in the end?
There seems to be a small leek from the Geberit outlet valve into the toilet bowl so that it constantly dribbles and it has to keep refilling.
Everything seems to be tight and sealed.
There is a blue...