ISOCGI - enable multiple documents on single Ustrd tag line

We are looking to use ISOCGI Credit Transfer 001.001.03 Unstructured as a base for Swedish and Swiss ISO payment files.  Problem we have is that if there are multiple documents being paid a separate Ustrd tag is created for each, I can't find documentation to tell me how to create 1 Ustrd tag line which contains multiple invoice details.  Both the Swedish and Swiss banks won't accept multiple Ustrd tag lines in the file. Only solution at present is to do 1 document per payment but this prevents us including credit memos.