The police officer leading the murder hunt said he understood why the public was frustrated that no-one had been arrested for the crime in the six months since her death.

But Detective Chief Inspector Hamish Campbell said officers were "confident" that those responsible for her brutal shooting would be brought to justice.

Crimewatch presenter Miss Dando, 37, was shot once in the head on 26 April, the killer or killers striking on the doorstep of her home in Gowan Avenue, Fulham, west London.

An organised contract killing or a stalker obsessed with the presenter remained the most likely murder scenarios, said DCI Campbell.

'Inquiry bill at �2m'

The police investigation, which will have been running for six months on Tuesday, has now cost more than �2m, it has been reported.

The bulk of the bill has been run up because of the large number of officers sifting through the evidence and mounting surveillance operations on suspects, The Sun newspaper claimed on Saturday.

Scotland Yard said it was not prepared to comment on the cost of the murder hunt.

In a statement Scotland Yard said the limited forensic evidence at the murder scene, just one bullet and cartridge case, always made it likely the inquiry would be "difficult and protracted".

DCI Campbell said: "We do accept that after six months people are wondering why we haven't charged someone for the murder of Miss Dando.

"What I want to say is that we have a number of clear lines of inquiry which we are rigorously pursuing.

"I cannot go into details for obvious reasons, but it is important that people realise that we remain confident that we will arrest and charge those responsible for the murder."

Four 'mysteries'

He added: "I firmly believe that there are people who know something about this crime.

"The killer would have had help to secure and dispose of both the vehicle and the firearm."

Police hope the distinctive bullet will yield a breakthrough

Public help is desperately needed to enable officers to clear up mysteries in four areas of the case, said DCI Campbell.

Around 50 officers were still working through the evidence, and had spoken to more than 2,500 people and taken some 1,100 statements.

And some 500 names had been given to detectives as possible suspects, and these were being systematically checked and eliminated according to priorities, added the detective.