Sawoniuk wrote the letter to Nikolai four years after he had settled in Britain. His half-brother was still living in Soviet-occupied Belarus and the KGB routinely intercepted all mail from the West. The letter revealed the whereabouts of a man the Soviet secret police had been looking for since the end of the war.
Sawoniuk came to the attention of the Russians as soon as they liberated Domachevo in July 1944. As was the practice, the Red Army held an inquiry to find out which people had sided with the Nazis. Sawoniuk's name was one of those quickly added to the Russians' list of wanted people. They looked for him but there was no trace. By this time, Sawoniuk had long since fled, crossing into Poland with the Germans in the company of his second wife, Nina. He then signed up for the Waffen SS but in November Sawoniuk deserted - dumping the pregnant Nina - while in Alsace, France. Perhaps having seen the way the war was going, Sawoniuk changed sides - signing up to join the 10th Hussar Regiment of Polish Corps. Intriguingly, Szymon Serafinowicz, who was also charged with murdering Jews but whose trial in Britain collapsed when it was found he was suffering from Alzheimer's disease, also served with this regiment.
Settling in Britain, Sawoniuk felt able to write to his brother the letter that would be intercepted. The Russians felt unable to act on it until in 1986 they passed to the Foreign Office a list of 97 suspected war criminals living in Britain.
At the same time, the Simon Wiesenthal Centre - the Israeli organisation that traces war criminals - had been pushing the British to act against suspects. The head of its Israeli office, Dr Effrain Zuroff, was working on lists of suspects living in Britain, Canada and Australia. In October 1986 a list of 17 names - not including Sawoniuk - was presented to Britain.
The persistence of Dr Zuroff and others led to the establishment in 1988 of a War Crimes Inquiry, led by Sir Thomas Hetherington and William Chalmers. The Russian list of 97 names, among them Sawoniuk, was passed to this inquiry. Hugely controversial, the inquiry recommended that the issue of war criminals living in Britain could no longer be ignored. In 1991 Parliament passed the War Crimes Act, which extended the jurisdiction of British courts to deal with crimes committed outside the United Kingdom.
But Sawoniuk is likely to be one of perhaps only two people prosecuted in Britain under the act. Police have revealed that of the 376 cases of alleged war crimes they have investigated since 1991, only one other possible prosecution was pending. Another 30 suspects living in Britain against whom there is evidence of war crimes have been diagnosed as being too ill to prosecute. Another 117 have died. In the other cases the allegations were discovered to be unfounded or else there was insufficient evidence.