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.

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