I have now slightly amended the code to :-
$default_image = update_field('_thumbnail_id_pyjamas', 400);
//$values = get_fields( $post_id );
// var_dump($default_image);
//die('VALUES');
$data = array(
'ID' => $post_id,
'post_title' => $title,
'post_name' => sanitize_title( $title ),
'_thumbnail_id_pyjamas'=> $default_image
);
Still no joy.
Can anyone see what I am doing wrong?
Thanks