Five years ago next week, Madeleine McCann disappeared from a family holiday apartment in the Algarve, Portugal. Why has the little girl, aged three when she went missing, never been found? Madeleine's parents, Kate and Gerry McCann, are adamant that their daughter was abducted. Frustrated by Portuguese police and public opinion that viewed them with suspicion, Madeleine's parents persuaded prime minister David Cameron to order a review of the case. Is this new investigation, by a top unit of the Metropolitan Police, the last chance of discovering what happened? For the first time, the senior UK investigator tells Panorama how he is working collaboratively with Portuguese police and explains why he believes he has the best opportunity yet to help solve the mystery of Madeleine McCann.