Hi James, yes I solved this a couple days ago, just been too busy to post.
I ended up with this:
<?php
$temp_post = get_post($post_id);
$author_id = $temp_post->post_author;
$background_main = get_field(‘background_image’, ‘user_’ . $author_id);
?>
background-image:url(‘<?php echo $background_main; ?>’)
