Alerts Composer - recipient as Assignment manager (not person manager)

Summary: Alert needed via composer to assignment manager, not person manager.

Content (required):

We have a large number of multiple assignments - lots with differing managers for the same employee. We need to send a notification (which is a reminder to the manager that their employee's assignment is nearing it's due date) to the manager, but it must be the assignment manager. Not the person (or primary) manager.

I've tried some obvious options, and even managed to get it to work for the assignment's line manager +1 (manager's manager) with the code below. Must be missing something obvious (I hope I am!). Thoughts appreciated!