Cutting a birthday cake iced with “Back Boris”, the man likely to become prime minister next month noted with satisfaction: “It’s blue all the way through.” The frontrunner in the Tory leadership contest then gave a brief pep talk to 20 or so keen young volunteers crowded into the basement of his campaign headquarters yesterday, his 55th birthday. Not the meandering, joke-strewn stump speech full of surprises that his fans adore, but a focused and serious message. “We are by no means there yet,” he said, warning that the next phase of the Tory leadership contest will be the toughest. “This movement is going to have to build.” So has the old Boris been retired? The one who made Londoners and the nation laugh with him (and sometimes at him) as eagerly as they voted for him? Giving his first interview to a daily newspaper since the campaign began, Mr Johnson was clear that he is no longer playing for laughs. “Well, I do think it is time we had some excitement back into politics, I do, but there’s also quite a serious job of work to be done.” The former London mayor was sitting in the study of 11 Lord…