Five years ago, Madeleine McCann disappeared from a holiday apartment in the Algarve, Portugal. Why has the little girl, aged three when she went missing, never been found? Her parents are adamant that she was abducted, but frustrated at being treated with suspicion by Portuguese police, they persuaded Prime Minister David Cameron to order a fresh review. Is this new investigation by a top unit of the Metropolitan Police the last hope of discovering what happened? For the first time, the senior UK investigator speaks to Panorama and explains why he believes he has the "best opportunity" yet to help solve the mystery of Madeleine McCann.