Windsor Davis (Williams) and George Layton (Solomons) both previously appeared on Doctor Who during the 1960's although not in the same story.
Originally Written as a one off Television play, it proved popular enough to be expanded into it's own TV series, hence this episode lacks the usual motif of Sgt Williams mistaking Private "Parky" Parkinson as his son, an idea that wasn't introduced until the pilot was expanded into a series.