I've read that. It's one of the Make Your Own Adventure With Doctor Who books, so it's not canon or anything - which is just as well, because as a serious Doctor Who adventure it rates somewhere in the vicinity of the old TV Comic stuff. It never does explain what Turlough's doing travelling with Six - I kind of got the impression that the author had written it for Five, and then there was some hurried rewriting so that all the MYOA books would have Six.