Solved: This turned to be a really simple fix without any data connections. To display Forms library columns in the InfoPath form, I did the following (Please excuse any ambiguity as this is from memory).
1) Created a number of fields, on each, under 'display' ticked the box to make these 'Read Only'
2) Published the form and under the 'Property promotion' made sure that all of these columns were able to be edited in Datasheetview. (This is needed so that the workflow can update the columns).
3) When publishing the form, I made sure that this created a new Document library. (This didnt seem to work when saving as a content type) and created new columns in the library.
I then simply amended the Nintex workflow so that when a new Item was added to the forms library, this run the workflow and updated the relevant columns.
4) When I reopened the form, these were now showing the data from the newly updated columns.