Well, I was away last week and didn't quite get around to requesting a couple of books (and making an ILL request) until last night.
as you can see it didn't go quite as smoothly as I hoped.
So, this afternoon, after collecting our mail, I popped into the library and showed them a screenshot of the error message.
It turns out that the library system automatically marks you as an inactive user after a couple of years if you don't use it regularly - and my last interaction had been back in 2018.
None of this was a drama when you live in a small town - they just fixed it there and then, leaving me free to go and make my hold requests ...