http://www.alycidon.com/ALYCIDON%20RAIL/INFORMED%20SOURCES%20ARCHIVE/INF%20SRCS%202003/Informed%20Sources%2001%202003.htm
If you read that link you will see that Virgin wanted 7 car DMUs to replace the HSTs. The Voyager fleet was however specified by the DfT to be only 4/5 cars in length...