To be fair to Harry and Lady Di, the red hair genes are autosomal recessive and can stay in the background for several generations until both prospective parents happen to be carriers, and even then there's only a one in four chance of their child being a redhead. To me, Harry's facial features do resemble Charles's to an extent. If Hewitt was Harry's dad, then Diana would also have been carrying the genes, suggesting that the genes were in her family anyway, and were just waiting fo their chance to pop out...

Genetics was always my weak spot at university, so I stand to be corrected...