Business Green: London Mayor Boris Johnson has waded into the escalating row over the proposed third runway at Heathrow, pledging to make a contribution of £15,000 towards a legal challenge if the government grants approval to the controversial expansion plans. Transport secretary Geoff Hoon is widely expected to give the go ahead to the plans next month, despite a threatened backbench rebellion on the topic and widespread criticism that expansion of the airport will undermine the UK’s climate change …