@Kain_W Thanks for replying but this does not help, and I've seen the same advice in other threads on this matter. Nobody seems to report that this has fixed the problem. Perhaps I can make it clearer, and maybe somebody with some technical expertise on whatever is going on in the VM backend can assist.
This is what my VM account says:

So Netflix is "active". Incidentally, the renewal date changes to whatever today's date is. If I click "manage Netflix" I get this:

So, no use really. It's just a link to the Netflix login page. Now, if I try to sign in on the Virgin TV box, I get this:

Entering my mobile number just prompts me to reactivate my old Netflix account, which I allowed to expire because I'm supposed to be getting it included in the Virgin package. The only option in the next stage is to add one of the three standard Netflix payment plans, using the Paypal account Netflix has on file. There's nothing about billing via Virgin.
Other people who have had this issue solved report receiving an email from Virgin, with a link to click, and then a few days later a second email arrives with Netflix activation details. This is what the customer services person told me I'd be receiving "shortly" when I did live chat last Saturday. He told me he'd raised a request for this email to be sent out. So, presumably, that is what I need.
The O2 phone part of my Volt bundle came with a 6-month sub to Disney+, and the difference in signing up via O2 was, well, incomparable. It took about 30 seconds. I've been trying to get this thing working for over a week, and judging by the comments on these forums I'm not alone.