When the Conservatives won the 2015 election, David Cameron reiterated his manifesto promise to hold an in-out referendum on Britain's membership of the EU. Not long after, the starting gun was fired and the battle lines drawn. What followed? It's a well charted path; but Mark Mardell talks to the people who were there at the key moments, to see what they think the defining factors are, and whether they think things could - should - have been done differently. Email: worldatone@bbc.co.uk. Twitter: @BBCWorldatOne