There are indeed nine preserved. What the program was referring to, which they really didn't make clear, was that 92134 is unique because it is the only single chimney example left. The original design had a normal single chimney, but BR experimented with various modifications to improve the locos further; one of these to try a double chimney on 92178 I think. This turned out to be the only experiment which was an unqualified success so all future new-builds were built with a double chimney and some were fitted with them later. 91234 is the only single chimney survivor.