Thank you Daestrum, I am obviously not very good at reading instructions... I had downloaded Open JDK separately while reading about Java, I overlooked the blurb about JavaFX in the B4J installation instructions...
Update: I downloaded the full package linked by Erel and it is working. It is 11.0.1 instead of 11.0.2 as I downloaded from the OpenJDK site but it seems to work just fine. Thank you!
I am separately downloading the JavaFX package and I will try to add it to the 11.0.2 package for completeness.
Update 2: this is working too. I am the proverbial happy camper
Hopefully it will keep the corporate IT copyright enforcement police away from me re. Oracle...