Thanks John.
Got it, works a treat.
add_action( 'pre_get_posts', 'product_rmreference_column_orderby' );
function product_rmreference_column_orderby( $query ) {
if( ! is_admin() || ! $query->is_main_query() ) {
return;
}
if ( 'reference_id' === $query->get( 'orderby') ) {
$query->set( 'orderby', 'meta_value' );
$query->set( 'meta_key', 'reference_id' );
$query->set( 'meta_type', 'numeric' );
}
}