Managed to find a solution by building my own functions, I will explain the fix below:
The order of the ACF Gallery is stored in the SQL database table ‘wp_postmeta’, under the meta_value of (in my case) portfolio_gallery. There is an array of Image ID’s in the order you have saved them in. I had to gain access to this array, loop through the values, find the next and previous ID’s of the required images and output there URL’s via WP’s function get_permalink();
I will post my final code when I’ve had time to neaten it up 🙂
Hope this helps anyone else who finds them self in the same situation.
