To this end I've created a custom post type to hold it. It will work by using a combination of my custom posting form and the mail generation code. I create the newsletter as though it was a normal post, hit submit on the form and it then takes the content, transposes it for the mail and automatically sends it to the list.
It's actually a lot simpler than I imagined and I've already done all the hard work. I just need to build the form (which will be on a separate page template) and repurpose the mail generation code.
If this became a thing I would probably need to stop the daily post emails going to the same list, as this would be overkill, or provide opt in choices.