I've just notices this there are 5 people in the admin role, under permissions people in the admin role cant
Masquerade as user
Masquerade as admin
and in admin/config/people/masquerade the administrator checkbox is checked.
But only the admin account 1 can switch users the other admins get a the message "Access Denied, your not authorised to access this page" even though they do (or as far as I can tell) have the correct permissions set. Their permissions arent any different to uid1.