IIRC the main flyover routes opened Jan 2015, but speed restrictions were in place for most of that year while work continued track side, and the main and relief feeder routes were finished.
I think there was a Network Change Notice (NCN) at the time that dealt with the final line speeds becoming available; finding it on Network Rails useless website could take ages though...
PS. From another forum I found it was NCN Stage Q of the upgrade, to bring in the final line speeds through the station, that was supposed to have happened by early Sept 2015 (possibly after the August bank holiday). I can’t be sure of the exact dates.