Dan travels to Oxfordshire to meet Penny and her mother, Audrey, who by now is in her nineties. He hears how Audrey and her late husband, Charles, painstakingly tracked down information about Chris and Peta's whereabouts and made appeals on radio and TV.

Back in Greater Manchester, the now retired detective, David Sacks, talks about the investigation he conducted, with the help of Charles Farmer, that centred in on Silas Duane Boston. It's his file, sent to the US in 1979, that Boston's lawyer, Lexi Negin, says contained all the details necessary to frame her client. But were details in that file ever leaked? Dan tracks down the officer in the US who received it, Jim Kelly, to find out.

It's in speaking to Jim Kelly that Dan learns a startling revelation that explains why the case went cold for so long. And it appears Vince and Russell's former step mum, Kathe, may hold the key to closing out Lexi's defence - or maybe there's another way of solving this case.

Find out more about the case here: www.bbc.co.uk/paradise