I have found a solution.. well in fact it is more of a workaround.
1. On my web hosting provider’s systems I created a complete new WordPress instance. This was totally “vanilla”, with no plugins or themes (just the standard install theme).
2. I exported the posts from the main site using the built-in export function in WordPress.
3. Imported the posts into the temporary WordPress site created at step one.
4. Pointed the Wix WordPress import process at the temporary site.
5. Sat back for 15 mins or so while it imported!
Job done.
This is a potential solution for others if all else fails. It actually only took 30 mins or so once I came up with the idea.