How can I create a link from an organic groups content node back up to the parent OG?
burnsjeremy was right that the information I need is available to my node itself. To get this into a views block to position in a different place etc I can use contextual filter on 'Content: Nid' and 'When the filter value is NOT available' set the 'default value' to 'Content ID From URL'. I can then add the fields as normal via views.

Next will be the challenge of getting the parent group of a child group when on the node of a piece of content within the child group.