Why would my import of multiple selections work in one content type but not the other?
Eventually finding no solution to the problem I decided to replicate the multiple field selection import that was working.

I made a new content type which contained nothing but the data items I wanted to be able to select and then used a node reference from the original content type. This isn't the most opportune method as whenever this data is displayed it is displayed as a link, a link which then takes you to a page which has nothing on it apart from the name of the data. However imports correctly put the desired data into the the field.

My suspicions regarding this issue are around how the node reference handles multi-fields. Perhaps handling them in a similar way to the import, as in [datafield1,dataf2,"dataf3,dataf3,dataf3",dataf4](as clarification, the previous example is how the csv is laid out). However I've no idea whether or not this actually has any grounds.