As an update for anyone else that may be following this due to being in an similar situation (or just for recreation), the fourth PM I received (the first three where purely to get through security) was:
Thank you for clearing security.
I have spoken with the team who have advised that they will send out a new password for the portal so you can access your documents.
Have you received this?
and after stating that I had NOT of course received it, I got:
Thank you for the update, so I spoke with the team and they have advised this:
Please email DataSubjectRights@virginmedia.co.uk and quote [REDACTED-REF] in the subject, and they will be able to get this new password sent out.
Imagine my surprise then (not) this morning when I received a text message with a new access code to access the zip file (that I'm unable to download) and two new emails, one in my old NTLWorld inbox (as before) with the PDF instructions that explain I need to create a Kiteworks account and another in the same NTLWorld account junk folder (as before) with the Kiteworks link. Needless to say, attempting to use it and create the account as instructed yields to all too familiar:
I did notice that on the initial Kiteworks sign in page where you're supposed to click to create an account, there's an option to "Login via Single Sign On!". Clicking on that does indeed eventually give an option to sign in using a Virgin Media account. I thought perhaps this was the magic (i.e. ignore the incorrect instructions), but alas this only yielded:
Don't worry. I'll just add this addition time wasting exercise to my claim for a "Time and trouble award" when this eventually gets to the ombudsman/court.