Reviewer Rob - Escape ID before using it as a regexp (since + is a special character)

[As title]

Tested while cutting Halo release.
Still worth at least a comment, given that this breaks if we ever hook the live tests up to something that's not Ellis (not unheard of), add multiple domain support to Ellis (so I could have 12345@clearwater.org and 12345@clearwater.org.uk) or introduce usernames that aren't exactly ten characters long?