He then went on to assault two sex workers in Plymouth within days of his acquittal, and would later confess to murdering Class during an undercover police operation staged in 2010.
Following a seven week trial in 2021, also held at Sheffield Crown Court, he was convicted of both murders, and sentenced to life imprisonment with a minimum term of 37 years.
Between January 1982 and July 1983 he spent two terms at Baynard House, a residential home in Hull, where social workers noted a "split personality" where he could be well behaved one minute, and become violent the next.
[3] After joining the British Armed Forces, Allen was stationed in Germany, where, aged 19, he was reprimanded for attempting to steal an advertising flag and for causing criminal damage.
His violent temper continued, and on one occasion he threw a television through a window at the camp because Sonia's "Better the Devil You Know" did not win that year's Eurovision Song Contest.
[6] Samantha Class was a 29-year-old mother-of-three from Kingston upon Hull, who had been placed in care as a young girl, and who had become involved in sex work as a teenager.
[7] After disappearing on 25 October 1997, her body was discovered on the banks of the River Humber near North Ferriby by a group of schoolgirls the following day; she had been strangled, badly beaten and run over by a car.
[8][9][10][11] Allen was charged with Class's murder after his fingerprints and DNA were obtained during the routine investigation of an unrelated drink driving offence and discovered to be a match.
[8][12][13] The investigation, led by Detective Chief Superintendent Ken Bates, also involved an appeal for information on BBC One's Crimewatch and the offer of a £5,000 reward.
Lisa Welton, a former crime reporter for the Hull Daily Mail, has described how he was dropped at Sheffield railway station with some money and told to "get as far away from Yorkshire as possible".
Thirty-five days after his acquittal, he assaulted two sex workers in Plymouth, and was apprehended after one of his victims fought back and a nearby police officer heard her cries for help.
[4][16][18] In 2002, he spoke to a probation officer and confessed to harbouring a hatred of sex workers,[15] and described fantasies in which he wanted to hurt them: "I like to frighten them...
[21][22] When Allen returned to Humberside, police feared he may strike again, and launched an operation to assess his potential risk to sex workers in the area.
Operation Misty involved seven undercover officers and was designed to facilitate a potential confession by placing Allen in a situation where he might discuss the crime.
He told "Ian" he had committed house burglaries, had assaulted a police officer, and on 6 December 2010, admitted to killing Samantha Class.
[8][23][24] In the recorded conversation he outlined the circumstances leading up to the crime, describing how she had become angry and demanded money from him after the condom they were using had split during sexual intercourse.
Undercover officers eventually gathered over 400 hours of recordings, but Operation Misty was discontinued in 2011 after Allen attacked a sex worker in Scunthorpe.
Allen was arrested in January 2019 for breaching a Sexual Offences Prevention Order, which prohibited him from having contact with sex workers.
Police considered arresting Allen for what is termed a "bodiless homicide", where murder is suspected but there is no body, and began the process of gathering enough circumstantial evidence to link him to the case.
Testimony from Allen's flatmate at his previous trial was read out, describing how he had taken his clothes to a laundrette the day after Class disappeared.
Evidence was also presented from the Humberside Police operation during which Allen had confessed to the murder while speaking to an undercover officer using the name Ian.
After killing her, the trial was told Allen had used the internet to search for trowels and shovels, and downloaded Google Earth in order to look for a suitable location to dispose of the body.
GPS was then used to ascertain that he had visited his chosen location the following day, before taking the body to the Old Slaugh site overnight on 28 December, where he covered it with gravel.
He said that he had spent much of the evening drinking at the Holderness pub before picking her up in his car, and claimed the pair agreed on a price for sex before driving to Walker Street, where they engaged in sexual activity.
"[40] But Niki Adams, of the English Collective of Prostitutes, suggested the Grlakova murder could have been prevented: "This does feel like an avoidable tragedy yet again other women have gone on to lose their lives because the violence of a particular man was not addressed properly at the time and that is a pattern that we have seen over and over again.
"[41] On 23 June, presiding Judge, Mr Justice Goose sentenced Allen to life imprisonment, with a minimum term of 37 years.
[42] In his summing up, Goose described Allen as "an extremely dangerous man, with a long-held, deep-seated and warped view of women, particularly of sex workers".