Barry Johnston is Senior UK Political Advisor at Christian Aid. He writes in a personal capacity. Follow Barry on Twitter. As the David Cameron marched across the lawn to greet each of his G8 counterparts – firm of purpose, long of stride, open-neck of collar – his was the body language of a man about to embark on a serious charm offensive. His unenviable task, to woo an agreement on tax transparency from Obama, Putin and co, was made all…