Ms Hall was last seen at Cadillacs nightclub in Bath in the early hours of 9 June 1996 following a night out with friends.

Her remains were found by workmen on the M5 motorway more than 13 years later.

Detective Superintendent James Riccio, who's leading the investigation into the murder of the hospital clerical worker, said he is hoping for "one small piece of evidence" to find out what happened to Melanie that night and solve the case.

He said: "We will always be led by the evidence, but based... on the 6,000-plus investigative actions, 2,000 statements, the accounts, the intelligence, the deposition location, I'm of the belief that Melanie did know her killer."

As part of a public appeal in 1996, authorities released an e-fit of a man spotted inside the nightclub with a woman matching the victim's description and are urging anyone to come forward if they recognise them.

"This is still very much a focus of our appeal to the public today and we want to know who this man is," Det Supt Riccio said.

The force is also keen for information about an unconfirmed sighting of a woman, who potentially matched Ms Hall's description, engaged in an argument with a man in the Old Orchard area near the Cadillacs nightclub between 1.45am and 2am on the night of her disappearance.

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Detective Superintendent Riccio added: "We suspect that whoever has done this has told someone, and allegiances alter over the decades.

"Someone knows who killed Melanie and someone knows how she ended up at the side of the M5 at junction 14, and these dark secrets need to be brought into the light.

"We have a number of enquiries we're pursuing. It's very much an active case, and there may be more related operational activity in the near future."

Ms Hall's clothing - a pale blue silk dress, black suede mule shoes, a cream single-breasted long-sleeved jacket and a black satchel-type handbag - have never been found.

Some of her other belongings including a chequebook, bank card, a Next watch, and silver drop earrings are also missing.

Ms Hall's father Steve, speaking on a Channel 5 documentary, said: "When Melanie was murdered, it wasn't just one person's life that was taken, it changed and ruined many lives after.

"Mine, my wife's, my daughter's, my mother's - there's obviously someone out there, and probably some people out there, who know what happened to Melanie.

"If they felt they could come forward with information which led to us finding out what happened to Melanie, then at least we could feel that we've done all we can do for our child."

Police hope the documentary, The Body In The Bag: The Murder Of Melanie Hall, will spark memories and encourage people to get in touch with any information.