They are classified as novices, journeymen, dilettantes or masters. They are Britain's hitmen – killers who ply their deadly trade in return for cash, and who for the first time have become the subject of a major academic study.

The killers typically murder their targets on a street close to the victim's home, although a significant proportion get cold feet or bungle the job, according to criminologists who examined 27 cases of contract killing between 1974 and 2013 committed by 36 men (including accomplices) and one woman.

The publication of the research is chillingly timely: on Friday, high-flying City executive Robin Clark was shot in the leg in a targeted hit by a masked gunman as he got out of his car at an Essex railway station.

Using off-the-record interviews with informants, interviews with offenders and former offenders, court transcripts and newspaper archives, academics from Birmingham City University identified patterns of hitman behaviour in an attempt to demystify their secret world.

The reality of contract killing in Britain tended to be striking only in its mundanity, according to David Wilson, the university's professor of criminology. He said: "Far from the media portrayal of hits being conducted inside smoky rooms, frequented by members of an organised crime gang, British hits were more usually carried out in the open, on pavements, sometimes as the target was out walking their dog, or going shopping, with passersby watching on in horror."

Researchers found that the average cost of a hit was £15,180, with £100,000 being the highest and £200 the lowest amount paid. The average age of a hitman was 38 with the youngest aged 15 and the oldest 63.

The youngest, Santre Sanchez Gayle from north London, shot dead a young woman at point-blank range with a sawn-off shotgun in 2010 after she answered her front door. The oldest was David Harrison who, also in 2010, shot the owner of a skip-hire business in his Staffordshire home.

Most hits involved a gun, with three victims stabbed, five beaten to death and two strangled. The most conspicuous weapon was used in the killing of David King, a widely feared underworld figure known as "Rolex Dave", who in 2003 was shot five times as he emerged from a Hertfordshire gym by hitman Roger Vincent and his accomplice David Smith, both 33. The killing was the first time an AK-47 assault rifle – apparently belonging to the Hungarian prison service – had been used on a British street.

Most of the motives uncovered by the study were humdrum, often little more than a business dispute or a domestic falling-out between husbands and wives.

The most prolific hitman was someone few have heard of: John Childs. Between 1974 and 1978, Childs carried out six murders, including the killing of a 10-year-old boy shot in the head alongside his father because the killer became worried that he might be recognised.

The team also placed the killers into four distinct groups. The first was "novice" – those whose hit appeared to be the work of a first-timer – while the most able contract killers were classified as "masters" whose expertise was best encapsulated with the killing of "gangland boss" Frank McPhee in Maryhill, Glasgow, in 2000.

McPhee was killed by a single shot to the head only 500 metres from Maryhill police station with the hitman using a .22 rifle and telescopic sight. Like most "masters", McPhee's murderer was never brought to justice, which is why little is known about the characteristics and personalities of the most skilled killers.

"Indeed, might it be the case that there are some hitmen who are so adept as killers that the deaths of their victims does not even raise suspicion and are, instead, simply thought to be the result of natural causes?" said the study, which appears in the latest edition of the Howard Journal of Criminal Justice.

Another category was labelled the "dilettante", where the hitman had no criminal record, became involved often for desperate, usually financial, reasons, but was then unable to carry out the hit.

Jamaican legal clerk Orville Wright, 26, was defined as a dilettante after accepting £5,000 to murder Theresa Pitkin, 36. Wright burst into Pitkin's flat armed with a knife but after chatting to her decided he could not do it. The judge in Wright's trial described him as a "hitman who lost his nerve".

Another "dilettante" was Paul Cryne, 62, who in 2010 was jailed for life for the murder of Sharon Birchwood. Cryne had enjoyed a lavish lifestyle in Thailand throughout the 1990s after receiving a £500,000 insurance payout.

However, his excessive spending eventually caught up with him and while on bail for suspicion of carrying out another contract murder, Cryne managed to accumulate a debt of £11,000. He met Sharon Birchwood's ex-husband, Graham, in Thailand and accepted the contract to kill her for her life assurance payout. Cryne, who stood to collect £30,000 for the hit, flew back to the UK and strangled Sharon in her Surrey home. He was caught after leaving forensic evidence at the scene of the crime.

The other category was "journeyman", to describe contract killers considered capable and experienced but not as professional as a master.

The killers of "Rolex Dave" were classified as such. Although organised and possessing a formidable weapon, a lack of attention to detail meant they were caught – a plastic glove in the getaway car contained a palm print from Smith.

The only known female contract killer to have operated within this country was 27-year-old Te Rangimaria Ngarimu, who killed roofing contractor Graham Woodhatch in 1992. Ngarimu, a Maori living in London, was paid £7,000 to shoot Woodhatch while he was attending the Royal Free hospital in north London after an operation.

Ngarimu failed once to carry out the hit and returned to the hospital the next day, disguised as a man. She then fled to New Zealand. Ngarimu later returned to Britain to confess and in 1994 was sentenced to life imprisonment.