question regarding toilet cistern keeps filling

[pedro99]

I have a close coupled Ideal Standard cistern which fills up nicely but then water trickles slowly out into toilet pan, which then starts a top up of water beack to level in the cistern. Its not the overflow cos the water level isn't high enough. Its a Geberit E003167 flush valve Twico Replacement Accent Sotini Impuls 260 according to pic on tnet.
Any advice on why its doing this would be appreciated.
new to this so not sure if i'm in the right bit.

thanks : :

 

[Al-Glasgow]

Click out the twice valve using clips on side
Try turning seal around or replace, look for tiny bubbles on rubber seal, if u see any burst with pin.

Al