@LauraA You post an interesting question!
First I think it's important to distinguish the definitions of "fan art". The distinctions got a bit muddled during the podcast conversation
The question asked in the podcast was about "fan art". That is, artwork based on intellectual property owned by someone else (Batman, Captain Marvel, Star Wars, etc). In this case the fan-artist is looking at an image already created and then making a "new" image that looks substantially similar. In copyright law this is called a "derivative".
Fundamentally all fan art of this kind is an infringement. To legally reproduce it requires permission from the current IP owner.
As was mentioned in the podcast this fundamental infringement can (and often is) allowed/tolerated/sometimes encouraged by the IP owner for a variety of reasons that Jake covers in the podcast.
Now there's what you created for Lion the Witch and the Wardrobe . . .
What you made (while certainly artwork of a story of which you are a fan) is not "fan art"
The copyright to a text applies ONLY to the text and is owned by the author.
The copyright to an image applies ONLY to the image and is owned by the illustrator.
As you wrote:
I have liked the Narnia series since I was a kid, long before the movies came out. When I did my own illustrations I went out of the way to read the text carefully and not take my imagery either from the movies or from Pauline Baynes's lovely original illustrations. Any apparent similarity is either from the text or from my study of 1940s evacuee clothing
With this piece you've done exactly what every illustrator who has ever worked with an author has done for decades. The author can never claim ownership of the artist's visual interpretation of the written text.
You mention the "film producers". The film producers of LW&W created and own only their version of the story (the script) and their versions of the characters (actors likenesses and costuming choices)
Any similarities between your illustration and the movie interpretations CAN overlap but must be limited to the description in the original text. If your illustration looks significantly like the actor, or use a distinct costuming choice from the movie then you've crossed the line into infringing on the LW&W movie.
In this post I'm talking just about copyright. One of the fun new wrinkles for IP is that of trademark. I did see the there's a "word mark" held by the C.S. Lewis (PTE.) Ltd. for "The Lion the Witch and the Wardrobe". So if you were going to aggressively promote and sell the print I would suggest contacting an IP attorney