How do I update product attributes programatically on product save in Woocommerce

I am looking to update the attributes of the products in my store every time they are saved based on the content of several other fields.

However I am struggling to get my attributes to appear correctly.

Initially I tried to update the product based on the accepted answer to this question using the following function:

add_action( 'woocommerce_process_product_meta', 'update_dietary' );

function update_dietary( $object_id ) {
  wp_set_object_terms( $object_id, 'UK Manufactured', 'pa_dietary' , false);
}

This added "UK Manufactured" to the list of terms available for the "pa_dietary" taxonomy but it was not visible in the product attributes when I checked them.

Following this I tried a variation on the second answer supplied in the above question:

add_action( 'woocommerce_process_product_meta', 'update_dietary' );

function update_dietary( $object_id ) {
  $product_attributes = array();
  $product_attributes['pa_dietary'] = array(
      'name' => 'Dietary',
      'value' => 'UK Manufactured',
      'position' => 1,
      'is_visible' => 1,
      'is_variation' => 1,
      'is_taxonomy' => 0
  );

  update_post_meta( $object_id,'_product_attributes', $product_attributes);
}

This also failed to update the attributes for the products.

At this point I don’t know what to try next. Using the woocommerce_process_product_meta action seems to work, based on the fact that the term does get added to WordPress but I think there must be something I’m missing about how Woocommerce handles product attributes.

If anybody could point me in the right direction it would be very much appreciated.