Observer: The lush, muddy wetlands of the Tana river delta, teeming with birds and home to hippos and crocodiles, lions and elephants, are more than 4,000 miles from Britain. But this patchwork of savannah and mangrove swamp on the east coast of Africa is the latest victim of the British thirst for biofuels. To meet the worldwide demand and the regulation that, from next week, petrol and diesel sold in Britain must be mixed with bioethanol or biodiesel as part of a drive to cut the carbon …