Have just called Spark and was told that once an email address is terminated, it is possible for someone else to sign up and use that terminated email address - apparently this is in their terms and conditions.
Even the guy on the phone admitted this was a security issue but said anyone else 'can't see the history of the email' (not sure how that helps here).
I'm stuck wondering what to do as I certainly don't want to be paying for it, but I also don't want anyone else emailing me and it ending up in someone else's inbox.