I think that you'll be out of luck trying to find a DVD of the series. Only around one or two per cent (at the very most)of the BBC's output ends up on DVD, mainly due to rights issues. Unless there was already a reference to external distribution in the contracts of those involved, the BBC would have to negotiate individual contracts with nearly 50 different actors (or, to make it more complicated, with the executors or beneficiaries of their estates), together with Colin Welland's estate, etc, etc.
There's nothing on Youtube either (which, as VCRs didn't become common in the UK until a few years after that date, isn't particularly surprising).
As the open-reel video tape used by the BBC in those days was very expensive, it was common practice to wipe the majority of tapes, for future re-use, soon after a show (or a single repeat of it) had been broadcast. So the chances of the programmes still existing at all would seem to be quite low.
All that's left in the public domain seems to be the information on IMDB and the listings from the Radio Times:
https://genome.ch.bbc.co.uk/a1090fa790294df0862b3d781b3f1cb9
https://genome.ch.bbc.co.uk/f7571cd42ead40968032bc2566519f8a
https://genome.ch.bbc.co.uk/bb63ab22351648469fe6544ff99ce102
https://genome.ch.bbc.co.uk/30269616d7d142d9b737c88b0e347c2a
https://genome.ch.bbc.co.uk/0fd21b6416e54dac80b4036c9adacc2e
https://genome.ch.bbc.co.uk/4347b8b01acb476aa2513de2ba3c344b