He denied he was a killer and spent his days in court concocting ever more improbable stories to explain away the sheer weight of the evidence against him.

But last night William Kean spent the first night of his 22-year sentence behind bars after being found guilty of the brutal murder of Perthshire pensioner Jenny Methven.

His conviction brought an end to a case that shocked Scotland: that of a lively, content 80-year-old charity volunteer beaten to death in her own home in a part of the country unused to crimes of such viciousness.

Bludgeoned over the head with enough force to shatter both sides of her skull, Jenny Methven died just minutes after Kean launched his attack, and would have been unconscious after the second blow.

But while the shocking events that took place in her cottage in Forteviot on February 20 have been reconstructed by forensic scientists, the motive behind the crime remains a mystery.

A close friend of the family, Kean, a former special constable with Tayside Police, was described by Mrs Methven's son David as being "like a brother", and was one of the first people he called the day after his mother was murdered.

Kean, who ran an internet advertising business and a car valeting business, had been friends with Mr Methven for more than 20 years and knew his mother well, having carried out gardening work for her.

There was nothing to suggest a falling-out between the pair, and yesterday Mr Methven said the murder was "an act of betrayal".

Mary Kean, William Kean's wife, said the day of the murder had been like any other day, and that her husband and 12-year-old son had breakfast together before he set off.

But instead of going to work, he drove from his home in Blairgowrie to Forteviot, with his car caught on a newsagent's CCTV driving towards the village.

Mrs Methven was on the phone to her friend Elizabeth Cook when he arrived, and ended the call to her saying there was a vehicle at her driveway.

The phone would prove crucial in securing Kean's conviction, as his bloodied fingerprint was found on the receiver.

After murdering the pensioner in her house, Kean went home and later kept an arrangement to give his niece a driving lesson.

All witnesses called to the court agreed there was nothing strange about him on the day, but afterwards his behaviour became more extreme.

He attempted suicide in the weeks after Mrs Methven died, and left a note saying he was worried about being blamed for something he had not done.

He also cut his palms and fingertips in an attempt, according to police, to prevent them from being matched to prints found at the scene of the crime.

Kean claimed the murder was committed by Glasgow gangsters at the behest of David Methven, that he had been offered £30,000 a year for three years by Mr Methven in exchange for his silence, and that the murder was linked to drugs and money-lending.

But his most outrageous lies centred on his attempts to explain away other vital clues found in Mrs Methven's cottage. After it emerged his DNA had been found on a glass in the sink and on the victim's wrist, he said he had called round and found her lying in a pool of blood.

Kean said that he had left her there, even though she was fatally injured, and had been too frightened to call for an ambulance.