According to my standard Putnam reference - a database once available from the University of the West of Scotland - the second edition, which was subsequently reprinted twice, added 27 pages. Here are some comments from the database. I don't own the book myself.
"This updated and expanded edition contains more than 200 new photographs and an enlarged section of Vickers’ Projects. Since publication of the first edition research has enabled the list of Vickers Type numbers to be more complete, and the sections on the Viscount, Vanguard and VC.10 have been brought up to date. E.B. Morgan"
"The late C.F. Andrews … had access to a mass of unpublished material … This … has been augmented and updated by E.B. Morgan, who collaborated with Andrews on the first edition."