The most straight-forward approach from here is to insert this into a temp table, open a cursor on it, run through the data and modify the "Set" column so that the batch/invoice numbers are appropriately grouped together. Then join the results of that temp table back to the original data set via batch/invoice number, so that the modified "Set" column is appended. This cursor-based solution is here (the rules require a set-based solution, so I did the cursor-based one just as a baseline to compare to the set-based solution).
