Princess Spindrift

Does anyone remember a Jackanory story read by Michael Jayston over 5 days, called, "Princess Spindrift"? I believe it was first transmitted around September 30th 1976. It was the loveliest of fairy tales concerning the eponymous Princess Spindrift, whose mother was a mermaid and her father a king. Her cousin, Morgan came courting one day, driving up from the sea in his chariot driven by white horses of the waves. He is handsome and charming and wants to take Spindrift down into the underwater kingdom for the day. She begs her father who reluctantly allows this, but there is danger involved - she can only stay until sunset. Her father gives her a talisman (either a gold medallion or a whistle, I can't remember exactly). Once she has been taken to the underwater kingdom she becomes trapped in some way and has to resort to using the object given to her by her father. Unfortunately, my (then) small daughter and I missed the very last episode and never knew the outcome! Over the years, as books have been published containing collections of Jackanory stories, it has been very disappointing that this particular story, with its lovely illustrations has never been published. As far as I know, this story has never been repeated and it has been difficult - in the pre-electronic era - to find out anything about it. I did once persist with correspondence to a very unhelpful lady who worked at the BBC in the 1980s. Eventually she sent me the name and address of "E.F.MacDonald" and I wrote asking if this delightful story would ever be published, and asked for the name of the illustrator. Sadly, the letter was returned marked, "Deceased". I would love to find out more - particularly the artist who provided the drawings and of course, the outcome for Princess Spindrift, torn between her kingdom on land and the seductive Kingdom Beneath the Sea.