By Simon Mullock Manchester United have landed a huge blow in their bid to close the gap on rivals City by convincing Omar Berrada to quit the Etihad to become their new chief executive. Berrada has agreed to take the top job at Old Trafford after almost 13 years working on the opposite side of town, eight of them as the champions’ chief football operations officer. But Mirror Football understands he will have to serve a notice period that means he is unlikely to start work at United until the 2024-25 season has kicked off. Berrada, a Spaniard who is fluent in five languages, has been at the h…