Workbench moderation allowing a role to unpublish content?

Question

I'm using Workbench Moderation on a new D7 site build. All is great in terms of workflow and I think I have the permissions near perfect, but I can't for the life of me seem to grant my 'content approver' roles the ability to unpublish content.

If I log in as a administrator (Drupal default role) I can see the option to 'unpublish' appearing, but I can't spot a particular Workbench / Drupal permission to grant to allow this permission for lower roles.

Anyone have a suggestion?