Need help with unidentified part of toilet tank

[cport144]

Any idea where this piece that i'm holding goes? And will it help to stop the issue i'm having of water leaking from the tank into the bowl?

 

[rpm]

Am I correct in thinking that you`ve recently had somebody in to fix this, paid them but still have the same problem. If so then I`ll say it was replaced with the smaller and different valve in the top right of the photo so you can throw it away.

 

[Sparkgap]

1) Looks like old valve bit: chuck it. 2) How quickly does the water leak? The square bit acts as an overflow so might be a case of just adjusting the float on the new valve to lower the water level. I always find flap valves let a little dribble past but nothing to worry about.