Woah! Cracked it thanks for your help John – turns out because the images are in an array I dont need a nested array in update_field – so this works!
foreach( $imageurls as $url ) {
$carpic[]=
array('field_5e70d223200ab' => $url);
}
update_field( 'field_5e70d211200aa', $carpic, $post_id);
Knew it would be my fault 🙂
