Thanks very much for your help Jay, can you help me with a little sample code on how to unhook and hook it back again?
Maybe that's my problem, I have tried something like this with no success:
remove_action( 'woocommerce_after_single_product_summary', 'woocommerce_output_related_products', 20 );
... and then call it back again:
do_action( 'woocommerce_after_single_product_summary', 'woocommerce_output_related_products', 20 );