It's the 100th episode of Grand Designs, and to mark this momentous event - a truly momentous project. Leigh Osborne and Graham Voce want to convert and extend a landmark 150-year-old, 100-foot-high water tower right in the heart of central London. Grade II listed, but utterly derelict, with six foot thick walls and crowned with a huge steel tank - it is an almighty task. As if this isn't challenging enough, Graham and Leigh also plan to build two additional contemporary structures at the base: a lift shaft connected by a series of glass tunnels, and a modern living space called 'The Cube'. The result will be a four-bedroom property over nine floors, complete with a room at the top offering spectacular 360 degree views across the capital.