Hi Martyn,
At this time, you must submit a request to Support to add another
account administrator.
Please contact Support (information is located in your Google
Dashboard) with the following info:
(1) The email address of the new administrator
(2) The exact name of your top level user org
(3) The Organization ID of your top level user org (to verify
ownership). This ID is located in the Summary box for the
organization.
With this information, we can move ahead with creating another account
administrator for you.
Thanks!
Jennifer