I eventually received help with this from elsewhere but thought I would mention the solution here for anyone with the same problem in the future.
The problem related to the "4 fields from 4 related tables" so I had to bring those fields into the same layout that the report was based on which was the 'Site' table. I did this by adding a Calc field which simply pointed to the original Calc field in the related table.
I then added a Summary field called _sNewCalcField which has a 'Total of' the NewCalcField.
On the report itself, I added a field based on the new Summary field.