Composer generates the autoloader for you. Thus, you don’t need to require a specific file from vendor packages. All you need to do is require the composer autoloader:
Change require_once dirname(__FILE__) . '/vendor/Frozensheep/RightmoveADF/RightmoveADF.php'; to
require_once plugin_dir_path(__FILE__) . 'vendor/autoload.php';
In some cases, classes still not be found. Re-generate the composer autoloader by $ composer dump-autoload.
Usually, you should not mess with the vendor directory. If you did, I suggest you remove the vendor directory and re-install packages by $ rm -dr vendor/ and $ composer install