I am an Agatha Christie Fan and she is the only Actress who played Miss Marple on Television who I haven't seen, It was made in the 50s and also had Roger Moore in it.
well i have just tried, play.com, hmv, 101cd, amazon, and e.bay. and they haven't got it at any of them in dvd or video. so i guess that means its probably not on video or dvd, but by all means keep looking.
they probably just haven't got around to transferring it yet, or if its from the 50s they may not have the master tape to copy and is lost to posterity.