So we managed to work around this issue by turning on the web version and sending to our 3rd party, they were able to get what they needed from the web version for the HTML email and it worked perfectly, so if anyone is stuck this is a good option (unless the 3rd party you’re working with isn’t the most helpful - they may need to remove some code to make it work)
We found that copying the source code from the editor didn’t work for us