Content pane of users related to node by field using panels, views (D7)
The problem was that in the display fields section for my Content Type I had service_type hidden (because I only wanted to use it to filter Users without having to use a vocabulary), as such it wasn't featuring in the context and wasn't being passed into my View Content Pane.

I had to leave it displayed, then create a custom display for 'Full Content' and hide the field there instead.