According to the wikipedia entry for Rediffusion & Associated Redfiffusion http://en.wikipedia.org/wiki/Associated-Rediffusion , most of the rights of the surviving episodes are either with the British Film Institute National Archives
or with a company called Archbuild Limited who hold the rights to the compilations from which the DVD was made. The same applies to the companion series "At Last the 1948 Show".
I am located in Australia and we have had an identical problem with the archiving of locally produced television programmes. Because of the high cost of production of TV programmes, video tape recording as opposed to film recoding was seen as a blessing in disguise back then. It was seen as cost effective to re-record over a used video tape and I doubt whether anybody gave any thought to actual archiving of recorded material, particularly with ever-changing technologies.
We have seen over the last couple of years many compilation programmes celebrating the first 50 years of Australian television. Much of the archived material used was from film stock recorded in the 50's to very early 60's and therefore pre video tape recording. It is easier to get access to archived 1940's war-time news-reels than 60's and 70's TV programmes.
Ironically, it is also easier to get American television programmes from the 60's because so much of it was sold cheaply around the world that they never really stopped televising them. Australian television broadcasters repeated them ad-nauseum and continue to be seen to this day either on Pay-TV or on DVD releases. "F Troop", "McHale's Navy", "Bewitched", "I Dream of Jeannie" to name but a few. To get your hands on British (or Australian TV) programmes from this era is nigh impossible.
In a parallel situaltion, I am involved in radio broadcasting. We have a collection of old equipment in a store room that are museum pieces now. Reel-to-reel tape decks, D-Carts and 8-track cartidges, turntables, cassette decks, CDs and Mini-discs. Most of it is in working order, but nearly impossible to repair now because of the scarcity of parts. Over the years we have been archiving from one technology to the next and currently using MP3 recorders to record interviews in the field and accessing and creating programmes from a computer based database catalogue of our music collection. I suppose we have only appreciated the real value of archives in the last 20 years of our 30 years of broadcasting. No doubt today's iPod is tomorrow's museum piece, but the recoding will be archived to the next available medium.
Hindsight is a wonderful thing, it really is a shame we didn't have it 40 years ago.
Cheers and beers,
et tu Brute
Locked my wire coat-hanger in the car - good thing that I always carry spare keys in my pocket :)