I posted on the VirtualBox forum, and discovered that I didn’t have the latest Version of VirtualBox (I had thought that I had).
The latest version of VirtualBox (version 7.1.6) supports IPv6 out of the box — no special settings required!
This is a great relief.
VirtualBox 7.1.6 introduces a couple of new problems, which I’m busy trying to address, but at least I can work around them at the moment.
So, solved!