Detectives seeking the killer of Crimewatch UK presenter Jill Dando say an appeal for information on the programme prompted hundreds of calls.

The man heading the inquiry, Detective Chief Inspector Hamish Campbell, said they had received more than 500 calls from the public following Tuesday night's broadcast.

He said he was particularly interested in two calls from passengers on the number 74 bus, who overheard the prime suspect speaking on his mobile phone shortly after the murder.

The BBC's Stephen Cape: Detectives may have the break they needed

Mr Campbell said they still had no idea of the motive and he said: "We need to know who has gained financially from this murder after 21 days. Has someone gained financially or emotionally?"

Ms Dando was murdered with a single shot to the head outside her home in Fulham, west London on 26 April. Tuesday's edition of Crimewatch UK opened without titles or music and her former co-presenter Nick Ross said described the programme as "sombre and surreal".

He said: "It has been almost unbearable working on Jill's death."

'Passionate'

But he added: "This programme was her passion and now, as Jill helped others, our help can do the same for her, her family and her fiancé Alan (Farthing)."

Mr Farthing told the programme: "Somebody has murdered the person I had hoped to spend the rest of my life with."

He said: "It's always difficult for anyone to cope if you don't know why something has happened. It just defies logic."

Alan Farthing: "Somebody has murdered the person I had hoped to spend the rest of my life with"

Mr Campbell said they wanted to trace two men who had been seen in the Gowan Avenue area on the day of Ms Dando's death.

He said he believed the pair were probably different men but it was not clear if they were acting together. The prime suspect was a dark haired, heavily sweating man seen waiting at a bus stop in the Fulham Palace Road.

'Where did he go?'

This is a gun similar to the one used by the killer

He got off at Putney Bridge underground station and disappeared.

Another man was seen sitting in a Range Rover close to Ms Dando's home on the morning of the killing.

A traffic warden was about to give him a parking ticket when he knocked on the windscreen to indicate he was about to drive off.

Mr Campbell said he needed to speak to both men to eliminate them from the inquiry.

He said: "They probably think they were there innocently and can't be any help.

"But we have to find them. It will help us to ascertain the motive, by working out how many people were involved in her murder."

Detective Chief Inspector Campbell: "If he is a loner he may have had a fascination with Jill Dando"

Mr Campbell said they were keen to trace Range Rover drivers who were in the Fulham area on 26 April and he offered a guarantee of immunity for minor traffic offences.

He said it was still not clear whether the killing was a conspiracy or the act of a "loner".

'Does this sound familiar?'

"If it was a loner, maybe someone has noticed a friend or relative who had a fixation or fascination with Jill Dando. Was his behaviour different on that day?"

Mr Campbell also appealed for help on finding the murder weapon.

He said it was a 9mm "short" automatic, which was very different from the larger Browning gun.

Crimewatch UK's tribute to murdered crimefighter Jill Dando

Mr Campbell said the killer had used 9mm Remington ammunition.

At the end of the programme there was a tribute to Ms Dando.

'No hollow reassurances'

Nick Ross, wearing a solemn, dark suit, usually signs off the programme with his trademark "Don't have nightmares. Please sleep well."

He said "hollow reassurances" were incongruous in the wake of her death.

But Mr Ross added: "Her death was unprecedented. It was a one off and crime in Britain is still falling."

He said: "But we say it as we have always said it. Don't have nightmares. Do sleep well."

• Jill Dando's funeral service on Friday is to be relayed by the BBC to a park near the church in the presenter's home town of Weston-super-Mare, Devon, where it will take place.

The service and burial will be attended by just her family and close friends.

Thousands of mourners are expected to line the streets of the town. The hearse will follow a three-mile route.