The most powerful ad I created for David Cameron’s 2010 election campaign was a billboard featuring a still from a piece of BBC news footage. Incumbent PM Gordon Brown and Chancellor Alistair Darling had squabbled about something, so to allay suspicions of leadership disunity in the final stages of what had become a very close race, the two men co-hosted a press conference to show voters they were still in lockstep on the big stuff.