My original problem was finding out how to separate a file with 12000 rows into individual files with 1000 rows and keep the header. I found a great Q&A that does just that (with only one adjustment to number of rows), the link is here: How to split spreadsheet into multiple spreadsheets with set number of rows?
The only thing I need is for the script to save the files down as CSVs rather than Worksheets, but I can't see anything obvious in the script that defines the file type for me to change. FYI, I'm very new to this!
Thanks in advance