The socialist government of Venezuela presides over an economy in meltdown and a population desperate for change. Yet the country's opposition has failed to build a movement capable of bringing down President Nicolas Maduro. Why? In an exclusive interview, Stephen Sackur speaks to Leopoldo Lopez, the founder of the opposition Popular Will party. Last month, he escaped from Venezuela and found refuge in Spain. Is that the action of a man who has lost faith in the opposition's ability to win their struggle?