While fixing an issues with icons I discovered that the edit icon for assigning roles on the participants page was not enabled for the administrator.

After some further investigation I discovered the area where the problem lies.
user_can_assign() calls get_user_roles(). get_user_roles() does a search on the role_assignments table which administrators are not entered into. It finds no records and so no permission is given for assigning roles.

A change needs to be made in either of these functions (both in accesslib.php) to check for administrators.