Can't link my Steam account to my codemasters account.

I can't link my Steam account to my newly created Codemasters account (which is required for the Racenet club if I understand everything correctly).
The reason is it claims my Steam account is already linked: The Steam profile you are trying to link is already linked to a different Codemasters Account.

However this might've been done with an old Codemasters account on an email that no longers exist.

How do I link my Steam account to my new profile?