The BBC's Stephen Cape: "No immediate prospect of an arrest"

The appeal was screened on the programme, which Ms Dando used to co-present with Nick Ross, on Tuesday night.

Last month Crimewatch showed an extensive reconstruction of Ms Dando's murder on 26 April in an attempt to jog memories.

Mr Ross told viewers on Tuesday: "I was pessimistic after last month's programme.

Jill Dando...her murder appears motiveless

"It has been a hard slog, going through the calls, but Detective Chief Inspector Hamish Campbell says it may take several months to solve the murder but it will be solved."

Brother is confident

Ms Dando's brother Nigel has said he remains confident that the person who shot Ms Dando in the head outside her home in Fulham, west London, will be found.

"We're not at all despondent," he said. "I know it's been quite a long time, but we know the police have been working flat out on this inquiry, and that they've still got a mountain of information to sift through."

The attack on 26 April shocked the nation and seven weeks later obvious motives remain just as elusive.

Despite huge publicity, considerable resources and plenty of goodwill from the general public, the identity of the murderer remains a mystery.

A £150,000 reward has failed to penetrate the wall of silence surrounding the killer or killers. But senior officers are adamant the inquiry team is not being wound down.

They are working on two main theories:

• Ms Dando was murdered by a "celebrity stalker", or
• The killer had personal or professional reasons, which they have not yet ascertained, to shoot the former Crimewatch UK presenter.

Speculation

Over the last seven weeks detectives have been studying her lifestyle and interviewing former boyfriends and colleagues. Her brother is appealing to anyone who may be shielding the murderer to come forward.

The prime suspect remains on the loose

Senior officers say the investigation is so fluid a breakthrough could come at any time. But they accept it could go on for months and they can only speculate about the motive.

Suggestions that the killer may have been a Serbian agent acting in revenge for the bombing of a Belgrade TV station appear to have been ruled out by the police.

The BBC's Stephen Cape: "Senior officers are adamant the inquiry team is not being wound down"

Detectives also seem to have ruled out Ms Dando's former boyfriends, which leaves just the two theories.

The BBC's Crime Correspondent Stephen Cape says there are problems with the "celebrity stalker" theory.

Firstly, it would be completely unprecedented.

Secondly, the choice of weapon does not ring true. Offender profilers who specialise in the field would expect a besotted stalker, burning with rejection, to carry out a frenzied attack with a knife or blunt instrument rather than a gun, and the single shot suggests a more clinical motive.

However, former senior detective John Jones believes the killer is obsessed and proud of killing Ms Dando. Soon he will want to tell somebody about his "dark secret".

"By committing this murder in such a public way he was really advertising his feelings," he said. "And he will want to continue advertising his feelings.

"He will know that although at the moment the media is extremely interested in this crime, as time goes by the interest will die. He isn't going to want that to happen."

Meanwhile, police are believed to be honing in on the idea that someone hired a hitman to eliminate Ms Dando for some, as yet unfathomable, personal or professional reason.