Doomwatch has never had a complete DVD (or VHS) release. In fact, http://www.doomwatch.org/ Doomwatch.org is campaigning for one right now! The only DVD release (apart from the 1972 film) was a disc with “The Plastic Eaters” and “Tomorrow the Rat”. They had already been released on VHS and were unremastered. It was released in 2000, and is �50 (used) on Amazon.
I got hold on a VHS copy of the release 'The Red Sky / You Killed Toby Wren' (the other tape being 'The Plastic Eaters” and “Tomorrow the Rat”), and finally converted it to DVD. Its about �8 on Amazon, and about a tenner for the other one. Until there is a DVD release (and considering the stuff that does get released on DVD, you have to wonder whats stopping them), thats the best you can hope for.