The series was largely based on the book "Most Secret War" by RV Jones.
The first six episodes were broadcast in a block Jan-Feb 1977. The seventh episode, "Battle of the Atlantic", wasn't broadcast until Jan 1978: It wasn't titled "The Secret War", but used the same theme music as the original series and was narrated by William Woollard, and was included in both VHS releases.
The original VHS release was on four tapes - BBCV 4115 (Episodes 1 & 2), BBCV 4116 (3 & 4), BBCV 4117 (5 & 6), and BBCV 4118 (7).
The later VHS release was on three tapes: BBCV 5339 (1 & 2), BBCV 5340 (3 & 4), and BBCV 5341 (5, 6, & 7).
Up to April 2012, the series has never been officially released on DVD.
The original six episodes were released on Region 2 DVD in 2014 by Simply Media. (The additional program "Battle of the Atlantic" was not included.)