The case was investigated by officers from the Met's Fraud and Linked Crime Online Unit (Falcon) following a report by the victim and her daughter to Action Fraud in March 2015.
Alexander Bateman, of Headstone Lane, Harrow, was sentenced to two years' imprisonment at Wood Green Crown Court today, Friday, 10 February.
Acting Detective Sergeant Henry Childe, from the Falcon Unit, said:
"In stealing this priceless piece of WWII history through deception and lies, Bateman also deprived the victim of one of her lasting memories of her husband, an RAF war hero.
"Over the course of many years, Bateman went to great lengths to cover the tracks of his theft by insisting that the book was lost. He then gave the family false hope by informing them it had been recovered but avoided further contact, and he finally said it had been stolen in a burglary.
"He even tried to forge a Christmas card to make it seem like he had been given permission to keep it.
"Sadly, the log book has never been found and Bateman continues to refuse to tell anyone what happened to this unique artefact.
"Today's sentence is due punishment for Bateman's deplorable actions, and I hope that the victim and her family take some comfort from the fact that Bateman has finally been brought to justice."
Bateman, a keen military historian, contacted the victim - who by then had moved to Canada - in 1996 as part of his research into the RAF airmen involved in the 'Dambusters' RAF squadron. He asked if she could send him any photographs or log books belonging to her late husband and the victim duly sent her husband's log book to assist with his research.
Several years later, Bateman also contacted the victim's daughter who, upon learning Bateman had the log book, requested for it to be returned in January 2003.
An envelope arrived from Bateman, but it had been carefully cut open at the bottom. A piece of card and a letter from Bateman were found stapled to the inside of the envelope but there was no log book inside. Bateman initially claimed the log book must have been lost, but then said that he had recovered it from the Post Office.
His story then changed again and he told the family that he had been gifted the log book without condition, and later produced a Christmas card he claimed was from the victim, which had a note written inside apparently confirming his story of ownership.
Having stopped returning any calls, he then reported a burglary at his address in June 2003, where he claimed intruders broke into his home and stole the log book - a day before a national newspaper published a story about the dispute, which the family was then pursuing through a civil claim. However, the investigation into the burglary at the time never identified any suspects or forensic leads.
However, after the matter was reported to Action Fraud and then referred to the Met's Falcon Unit in March 2015, officers attended Bateman's address, where they found the Christmas card he claimed was from the victim. Handwriting experts carried out analysis of the writing in the card, and concluded that it was almost certainly not written by the victim.
Bateman was charged in July 2016 with theft and was found guilty of the offence following a five-day trial at Wood Green Crown Court.
Anyone with information as to the whereabouts of the late Sergeant John Fraser's missing RAF log book should contact Acting Detective Sergeant Henry Childe on 020 8345 4552 or call Crimestoppers anonymously on 0800 555 111.
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