... they did reply in the end, but not very helpfully, just suggested it was because of high volume of new signups (I don't believe that was the cause of the issue), and I could email my details through instead.
Then I had a hunch that my fairly long/complex password might be the issue, and it was!
I changed from a 29 character password to a 12 character one and joining worked. Not sure what made the difference, but hey ho, I'm a member now!