Detectives today said a £30 watch could hold the key to catching the murderer of a teenage prostitute.

They said Rebecca Hall was wearing an identical one at the time she went missing from her Bradford flat on Good Friday.

The 19-year-old mother's naked and badly-battered body was found in an alleyway near a notorious red- light spot 13 days later.

Despite finding other items such as her clothes at the scene in Thornton Street, Miss Hall's limited-edition South Park watch is still missing.

Detective Chief Superintendent Max McLean, who is heading the hunt for her killer, said: "Despite a very extensive search of the alleyway where Rebecca's body was found, the watch has not been recovered."

He added: "Obviously we are very interested in where this has been since the time Rebecca disappeared."

"It could be in the hands of the murderer or it may have been passed onto someone else in good faith."

He appealed for anyone who had seen it to contact the murder incident room, stressing that all calls would be dealt with in strict confidence.

Detectives are contacting jewellers who could have sold it or been offered it since Miss Hall's death.

After teams of forensic scientists examined the alley where Miss Hall's body was found - close to a spot used by prostitutes and their clients - officers carried out an intense search of the area.

"We cleared out the whole alley and sifted through every inch," said Det Chief Supt McLean.

Officers dug two foot into the ground, searching for the smallest clue. "It was a very thorough process - even to the extent that we found the butterfly catch to one of Rebecca's earrings."

Forensic experts concluded that Miss Hall had not been dead for more than four days when her body was found.

It is not yet clear if she was murdered and then dumped in the secluded spot or if she had been killed there and her body later discovered by a passerby.

One theory is that Miss Hall was held against her will before she was battered to death, and detectives are keen to hear from anyone who may have seen her between April 13 and 26.

Detectives have also enlisted the help of a 'Cracker'-style criminal profiler to help draw up possible characteristics of Miss Hall's killer.

Anyone with information on the murder should contact the confidential free hotline on 0800 318001.