I hadn't posted my question as a response to Toll Booth Willie's question, although it does look like some of the impacts and behaviours are the same. Which makes me hopeful that there would be some useful response. I'll wait and see.
I spent an hour on the phone with a Virgin Plus agent this morning, who, of course, recommended that I try replacing the modem. I've agreed to them sending me a new modem, only because my current one is over three years old.
However, I have my own modem connected to the Virgin Plus line, as well as the Valérie modem. As of last week, both modems have been assigned 10.xxx.xxx.xxx subnet addresses by the ISP. It therefore cannot be an issue with the hardware in my home, but rather an issue with the DHCP provisioning from the Virgin ISP.
10.xxx.xxx.xxx addresses are, by definition, local subnet addresses and are inaccessible to external connections. With these addresses, there is no way that I can access my home file server or photo albums. My personal mastodon server can broadcast my messages, but I can't access any of the posts of those I follow, nor see any responses to my messages. My personal website can only be accessed by me in my home. Half the apps on my phone (which must require two-way direct access) do not function unless I flip over to my (limited) mobile data. This is not the internet service I expect nor pay for.
So, once again.... How can I get the attention of anyone at Virgin Plus who has the capacity to make the changes to ensure that I get a true IP address, and not a limited subnet address?