Away from his respectable job and upstanding image, Coulton invited prostitutes to his home and kept a secret arsenal of weapons.

Coulton: Plotted murder

All this time he was guarding Princess Alexandra in Richmond, London. He and his wife Patricia would attend annual staff parties at the invitation of the princess.

In 1996 - after 13 years in Richmond - Coulton was transferred to guard duties at Buckingham Palace, He was based there until his wife was found stabbed to death on 10 January, 1997, in the grounds of the nursing home where she worked.

Killing fantasy

Christine Stone, Coulton's former lover and neighbour, told the court that the royal policeman had a fixation about her and had been thinking about killing his wife for years.

Patricia: Stabbed to death

Mrs Stone said Coulton fantasised about life without Patricia and told her that his marriage was a "sentence". She said their affair began in 1983. Coulton had talked about having children with her, she said, but she wanted to stay with her husband.

"He would talk about how he hated Pat and how sex was boring. He would often talk about killing her and how when she was in the bath he got feelings of wanting to drown her," Mrs Stone said.

"On one occasion he talked about doing something with the brakes on her car and he talked about electrocuting her while she was in the bath.

"There was also the fact that he wanted to set up something in the garage that she would touch and she would be electrocuted."

She added: "The more I wanted to end the affair, the more serious he got about killing Pat."

After their affair ended, she said Mrs Bridges said Coulton stalked her and checked registration numbers of cars outside her home.

String of prostitutes

Coulton was also seeing several prostitutes, while his wife was away at work as a care assistant.

One of them - Nirmal Salaria, who used the name Sherry - paid regular visits to his home in Woking, Surrey.

Coulton asked her if he could use her address as a mail drop for post that would come from Crawley Borough Council, where he kept a lock-up garage and any letters from the DVLA in the name of P Johnston.

He told Salaria that the car documentation was used in undercover police work. In fact they related to a Morris Ital car stored in the lock-up which played a crucial part in the murder inquiry.

The documents, handed in by Pc Pat Page who had been asked by Coulton to keep them for safe keeping, led police to the garage and a burnt-out Morris Ital.

Forensic experts found evidence linking the car to the orchard at Lynwood Nursing Home where Mrs Coulton's body was found.

Arsenal of weapons

Police also found firearms illegally stored at Coulton's house. He had acquired a shotgun from Princess Alexandra's chauffeur and sawed it down. But his lawyer, Christopher Sallon, QC, insisted that at no stage were any members of the Royal Family under threat.

Also found at his home was a Sten gun and a home-made point .38 pistol.

Mr Justice Wright sentenced Coulton to two years in prison for shortening the barrel of a shotgun, five years for possession of a single-barrelled shotgun, five years for possession of a Sten gun and two years for possession of the home-made pistol.