WHEN detectives arrived at a semi-detached house in the North Yorkshire village of Helperby late on a cold January night, little did they expect the case would create policing and legal history.

They found Paul Wilson, a manager of the pub next door, had died alone in the property and found him slumped over a toilet, where it is thought he desperately tried to be sick.

But Mr Wilson had left a trail of clues that would two years, three months later - after a unique and complex investigation involving witnesses from across the country and close collaboration with trading standards and crown prosecutors - lead to a restaurant owner to go on trial, charged with killing him.

The Northern Echo: A dish of chicken tikka massala

A dish of chicken tikka massala

Allergy Action founder Hazel Gowland said the case was "huge breakthrough" for the one-in-50 people in the country with a food allergy.

She added: "It is shocking that it has taken this long for someone to be prosecuted over an allergy-related death, but this time police managed to get all their ducks in a row.

"I am hoping this investigation will be a template for all future police inquiries of this sort."

After finding Mr Wilson, the detectives spotted a menu from the Indian Garden, in Easingwold, on the 38-year-old father's kitchen table, alongside a barely-eaten curry and a takeaway box, clearly marked with the words "no nuts".

Crucially, unlike previous allergy death investigations which were dropped before they reached court, they seized the curry. The meal was later sent for forensic examination by leading food analyst Dr Duncan Campbell, who found the meal contained a high level of peanut.

Within an hour of Mr Wilson being found officers were at the restaurant and, significantly, seized his order chit, which had "no nuts" written on it and CCTV from inside the restaurant showing Mr Wilson buying the curry. They also found of the four staff working at the restaurant, only the kitchen porter had a legal right to work in the UK.

The Northern Echo: Paul Wilson, who had a severe nut allergy

Paul Wilson, who had a severe nut allergy

The next morning, police alerted trading standards, who led a forensic examination of the restaurant's kitchen, while detectives were told by the restaurant's staff that Mr Wilson had asked for "no coconut".

Detective Sergeant Andy Palmer, part of a five-officer investigation team, said: "We then had the meal tested for coconut and it was found it contained lots of coconut as well."

A post-mortem examination concluded Mr Wilson had died as a result of consuming just 500 micrograms of peanut, two thousand times less than the weight of a paperclip.

Detectives then turned their attention to finding a motive for why peanuts had replaced almonds in the curry. They found the restaurant's owner, Mohammed Zaman, who had six restaurants in Easingwold and York, and a business debt of £294,000. They concluded this led him to reduce staff costs by employing illegal immigrants, forge a safety certificate and make no investment in staff training. Working closely with trading standards officers, detectives also found another of his customers, a teenage girl, had suffered an allergic reaction to a curry just 27 days before and had been warned by food safety inspectors.

When detectives spoke to a number of Zaman's suppliers, they found he had also been given warnings about the use of groundnut mix.

Det Sgt Palmer said: "Zaman was on both general notice from his supplier and on particular notice from trading standards over the earlier incident. His actions in using groundnut mix were exposing his customers with an allergy to an obvious and fatal risk."

Investigating officers found no food allergy-related deaths had been prosecuted, leading them to seek advice from the complex case unit of the Crown Prosecution Service.

Martin Goldman, chief prosecutor for Yorkshire, said it was immediately apparent the case was both unique and challenging.

He said a public policy decision had been taken to try to establish a criminal duty of care between the restaurant and the consumer to the standard necessary for a charge of gross negligence manslaughter.

To do this prosecutors found they would have to prove the actions of the individual were foreseeable, that there was a direct relationship between the supplier and the user and that it was fair and just to take the actions of people operating in "normal" society and classify that behaviour as criminal.

He said: "Gross negligence in crime terms means it's got to be more than merely bad behaviour and practice.

"There are an awful lot of people out there with allergies, so it was extremely important for them that we established this principle. What we had to do was go back to basics and build a case with police and trading standards."

As well as providing their food safety expertise for the case, trading standards officers ramped up their monitoring of restaurants across North Yorkshire, increasing test purchases for allergens from about two annually to 50.

Trading standards officer Andy Robson said through 2014 officers found a five per cent failure rate.

He added: "The last year has been more positive because there has been some new regulations brought in, unfortunately for Mr Wilson, they came in December 2014, and now there is more positive requirement of business owners to display allergens information to the consumer.

"Swapping cheaper ingredients for more expensive ones is starting to decline. It's starting to decline because people such as ourselves are being more proactive as a result of this case."