London's gardeners are twice blessed: not only do they live in one of the world's most vibrant capitals, it is also one of the most verdant. Gardens of every imaginable style, shape and size abound on rooftops, within palaces, surrounding churches, behind walls - on every piece of dry land - even if it is floating on or lapped by the river Thames. In Great Gardens of London, Victoria Summerley, Hugo Rittson Thomas and Marianne Majerus collaborate...