The two Metropolitan policemen who brought Hoogstraten to justice were a latter-day Sherlock Holmes and Dr Watson.

Detective Chief Inspector Richard Heselden is the thrusting, sharp-brained officer in charge and Detective Inspector Andrew Sladen is the older, pipe-smoking, more methodical type.

The pair collected clues and trod a winding and sinister road to the front door of the most notorious landlord in Britain since Peter Rachman.

Based at Shooters Hill police station in London, the officers relied on preliminary work carried out by Detective Chief Inspector Chris Horne, who relinquished the job on his retirement in October 1999, three months after Mohammed Raja's murder.

Hoogstraten's name was in the frame from the beginning of the investigation when Mr Raja's grandchildren, Rizvan and Waheed, saw two strangers attacking their grandfather at their home in Sutton, Surrey.

They heard shouts and went downstairs to see Hoogstraten's henchmen David Croke and Robert Knapp wearing disguises.

Mr Raja, 62, had already been stabbed and was holding his chest. He was shouting at the grandchildren in his native Punjabi: "These are Hoogstraten's men. They have hit me."

It was a clear indication of who was likely to be responsible but police had to throw their net as wide as possible.

Perhaps Mr Raja had recognised Knapp as an associate of Hoogstraten. Perhaps he just got it all wrong.

Mr Raja had a reputation for not looking after properties and perhaps an angry tenant had taken his revenge.

Messrs Heselden and Sladen had never heard of Hoogstraten before and, despite Mr Raja's accusation, they had no evidence against him.

As a routine inquiry, extensive research was carried out to find names of people associated with Hoogstraten and who might be capable of killing.

Police were excited when the name Robert Knapp emerged. He had used shotguns in armed robberies and had the guile.

Inquiries established Knapp was around when Hoogstraten and Mr Raja were embroiled in legal action in the mid-Nineties.

It was here that police found a motive.

Mr Raja had borrowed money from Hoogstraten to purchase properties for rent but Hoogstraten kept bank-transfer deeds as security.

Hoogstraten riled Mr Raja by collecting and pocketing rent from some of the properties - money Mr Raja claimed was his.

There was an ongoing dispute that involved upwards of £8 million and Mr Raja swore he was being threatened by Hoogstraten.

A "deep throat" informant, himself a suspect earlier in the inquiry, told police he knew Hoogstraten was behind the killing.

Police closed in but Hoogstraten remained cool. He appeared arrogant and composed.

Knapp and Croke were old lags, familiar with police and the interview routines. They showed little fear and said nothing.

The detectives were surprised at their reaction to questions about such a bloody and callous contract killing.

Mr Heselden said: "It was a particularly cold-blooded execution of an elderly man, who was stabbed and shot in front of his grandchildren in his own home."

Mr Sladen said: "It must have been incredibly upsetting for his family and we are eternally grateful to them for their patience.

"This was an extremely long and protracted investigation but they remained fully supportive throughout."

The detectives believe Mr Raja died because he stood up to Hoogstraten by refusing to drop legal action against him.