This one passed me by at the time, I think. I certainly don't recall it. I do recognise the writer's name - Clive Exton did a lot of sterling work for both BBC and ITV. Among his credits were two episodes of the Terry Nation post-apocalypse series Survivors, plus a slew of Jeeves and Wooster and a ton o' Poirot. He also worked on the script of The Awakening, the rather sluggish 1980 adaptation of Stoker's Jewel of the Seven Stars. I suspect Exton was one of those professional, reliable and highly creative TV writers who flourished in a very different production climate.
The imdb has the following bio:
Took his professional surname from Shakespeare's "Richard II".
Former English teacher at a comprehensive school in Cannock, Staffs.
Sadly, most of Exton's early work (single plays for the BBC) is now lost because the tapes were routinely wiped. It's not just Doctor Who fans who get frustrated by this BBC policy, rational though it may have seemed at the time. They wiped stuff, apparently, because they only paid for the music rights etc for two showings i.e. one repeat. Nobody anticipated a home video market that would reap millions for TV companies.