A month ago, Tom Watson received word that the Guardian was about to expose the hacking of Milly Dowler's phone by the News of the World. With 72 hours to go, he cleared his diary; a few days later, he was averaging three hours sleep a night, as he and his staff picked through leaked documents, newspaper archives, personal testimony from phone-hacking victims, and more. As the MP who had been obsessively trying to cut through the murk surrounding News International for two years, he well knew that the most dramatic chapter in the two-year phone-hacking saga had arrived – and the imperative now was to work harder than ever.

So how have the last few weeks been? "Sleep-deprived, totally crazy," he says, sitting in his parliamentary office during what seems to be a rare moment of calm. "But also, there's been a great sense of relief. I think I said something to David Cameron about a month before: that there were powerful forces trying to cover this story up. At some points over the last two years, I thought it might blow. But I've also thought that the lid could be welded back on. But when Nick Davies broke the Milly Dowler story, that was the point where I knew they'd never get the lid back on."

And has he been surprised by what's happened since?

"Yeah. I guess two years ago, I felt that all this would probably cost Rebekah Brooks her job. I thought the scale of wrongdoing was so great that somebody on the UK side of the company would have to take responsibility. And I was absolutely convinced that there was a cover-up. But I didn't know that it would all travel abroad. I didn't know it would get to America and Australia, and everywhere that it has." The closure of the News of the World, he says, came as "a genuine shock" to him, but he says that the same applied to News International: "There was a huge consumer boycott, there was going to be no advertising . . . I don't think they had a choice."

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