Britain´s steam-generating heavy-water reactor

In February 1975, the British Government gave consent for the construction of two new nuclear power stations: a 4-unit plant at Sizewell, Suffolk, for the CEGB, and a 2-unit plant at Torness in East Lothian for the SSEB. These stations will be the first to use the British-developed steam-generating heavy-water reactor following the long-awaited Government decision taken in 1974. Is the s.g.h.w.r. an entirely new reactor system? Is it unique? What are its special features?