Update a Case based on a Bug Status using Advanced Workflow

We are trying to gradually migrate across to Advanced Workflow, so I decided to try Advanced Workflow on this requirement instead of using the existing Workflow Module.

A bug has been worked on and the work completed and the Status changed to Closed. We want to update any Cases linked to that Bug and carry out the following:

1. Change the Status of the Case/s to a specific status

2. Advise the User/s assigned to the Case/s updated that they have been resolved

3. Advise Contacts linked to the Case/s that the bug has been resolved.

I hit a stumbling block at the first hurdle which surprised me and I am hoping it is my lack of knowledge.

The Process Definition has a Target Module of Bugs. 

Created a start event of Bug Status = Closed

Linked an Action to Update a field but when I tried to select the Cases Module I couldn't - only Bugs, Accounts, some User Links and I think from memory Opportunities.

Cases and Bugs have the standard many to many relationship so I am surprised I can't choose that module and select the field I want to update.

Am I doing something completely wrong - this seems to a pretty basic requirement.

Thanks in advance for any advice - we are not interested in 3rd party solutions.