Hi Jen, Ray, Graham, Dann
History of the Postal and Telegraph Censorship Department, 1938-1946, being a UK government publication, would have been covered by Crown copyright that lasts for 50 years from the date of publication (per
https://en.wikipedia.org/wiki/Crown_copyright#United_Kingdom) which was 1952. So, the publication fell into the public domain in 2003 and there should be no problem in reprinting. Unless you know better, they would not have a case against you if you made a copy from one of the CCSG printed versions. It would be about 225 double-sided pages.
Karl