Top Boris Johnson adviser Ben Gascoigne is departing, Guido understands. Gascoigne has been at Boris’ side for a decade, guiding him through both victorious mayoral campaigns, two elections and the EU referendum. Most recently he has been his SpAd at the FCO leading on parliamentary management and delivering on key policies. Gazza is well-connected and well-liked within the party and his departure will be a loss to the Foreign Secretary – landing him will be a coup for any private sector firm. It also opens up a plum SpAd role on Team Boris…