Paul

I ended up changing the domain I used a few years ago, because some stupid sites could not cope with a domain that had a '-' in it.

(i.e. they could not cope with "my-domain.com").

Unsurprisingly, Virgin Media were one of them, they did fix it eventually.
As I recall, they used a faulty javascript check, so I was able to bypass it and register.
[ Once created, I could log in fine, it was just the account registration that was broken ].