Thanks Giacomo Gelosi. These missions are getting harder!
First thing I tried after importing all the templates and workflows etc was to run it and the error message gave me a clue that it wasn't writing something correctly to the list. Incompatible data type or something so it was off to the workflow to check out what it did, where it was getting the data from and where it was writing to.
I noticed the Query XML was getting the Form Data to retrieve the repeating section and then a second Query XML was parsing that section. I tend to save my repeating sections to a multi line field - it's a bit easier to read and debug so that's what I did - off to Nintex Forms to save it to a multi-line field that I had created. I wasn't very creative with the name though - calling it "rs" for repeating section
Then I disabled the first query xml
and modified the second one to look directly at the XML
I put a few log to history commands in there to make sure the XML was being retrieved correctly which it was. I must brush up on the great article on Commit Pending changes because there were a few log items that weren't being displayed - something to do with create item. Putting one of those in before the create item action made sure I could see all my logging actions (now deleted from the workflow I've uploaded)
Awesome. I was still getting the error message though so had to delve further into the create list item action. There I found a couple of things - the main one being the syntax to add the first List item and a minor one with the date - not sure if that was intentional or not.