A few days from now David Cameron will arrive in Colombo to shake hands with a man who presided over the killing of at least 40,000 Tamil civilians and whose government continues to perpetrate shocking cases of rape, torture and mutilation – when the doors open on the Commonwealth Heads of Government Meeting (CHOGM) in Sri Lanka. As he sits down to watch the opening ceremony, the Prime Minister will not be able ignore the absence of two of his most prominent counterparts – the Canadian Prime Minister, Stephen Harper, and the Indian Prime Minister, Manmohan Singh. Both are boycotting the event.