As a rule of thumb, it’s always worth challenging an interviewee in the way that a political opponent would. So, when you get EU opponents on, it’s quite right to put the arguments that the pro-EU campaign makes: that Britain would be poorer outside, have less influence, blah blah. And the reverse applies: challenge EU supporters about the EU’s declining share of world markets, the absence of democracy in Brussels, the failure of the euro which, in many cases, the people you’re interviewing wanted to join. It won’t do to say: “But why are you so lamentably failing to convince people, Michael Heseltine?” Attacking Euro-enthusiasts from a pro-EU direction doesn’t count as balance.