My question is answered now, as the old contract finished last Friday and the new contract started on Saturday.
It turns out that if an email address is registered as a second user on an account then it can't be used to register another account. Also, only the primary user of an account can change the details of the second user. However, once the contract is terminated, the email address is then released and can be used to register another account. So, I just had to wait until Saturday and then my email address was available to register my new account.
I still can't see the point in registering a second user at all. My husband did it as he knew he would be working away most of the time and he wanted me to be able to manage the account. However, despite being the bill payer, I wasn't able to do anything on his account and customer support still wouldn't talk to me when things went wrong (which was the main reason for registering me). So, in the end, it just caused me more hassle when trying to set up my own account.
I won't make the mistake of registering a second user on my account.