A letting agent accompanying a plumber for a routine boiler service were shocked when they stumbled across a large-scale cannabis growing operation.

They also discovered one of those involved was a victim of human trafficking and has since been offered support.

Thamh Pham, 24, was politeness itself when he opened the door of the rental property in North Brink, Wisbech, on June 29 to the visitors.

But the plumber became suspicious of black bags in the utility rooms.

Initially he thought there was soil in the bags.

In fact, there were cannabis plants up to six-foot-tall sprouting from the bags with another man – later revealed as a victim -guarding them.

Police were called and Pham was charged with producing a class B drug; he admitted the offence before magistrates.

On August 26 he was jailed at Cambridge Crown Court for eight months.

Police estimate the 492 plants were worth up to £413,000 on the streets.

Pham, of no fixed address, answered “no comment” when quizzed.

Wisbech Standard: The scene that confronted police in Wisbech after a routine check by a plumber came across a drugs factory.The scene that confronted police in Wisbech after a routine check by a plumber came across a drugs factory. (Image: Cambs Police)

Police also found a large self-seal vacuum bag was also found which contained the flowering tops of a cannabis plant worth up to £7,100.

Further flowering tops were found elsewhere in the house worth up to £110.

DC Rhian Jones said: “As a result of the call from the plumber we were able to seize a large amount of class B drugs and stop them from being sold on the streets.

“A second man was also arrested at the house but later released.”

DC Jones said: “He was offered emergency accommodation and safeguarded as he was found to be a victim of human trafficking.

“Alongside seizing drugs, this is exactly why we target cannabis factories across the county.

“We can’t be everywhere and we rely on people in our communities being our eyes and ears. If something doesn’t seem quite right, it more than likely isn’t.

“We would encourage people to look out for signs of illegal cannabis factories in their communities.

Wisbech Standard: The scene that confronted police in Wisbech after a routine check by a plumber came across a drugs factory.The scene that confronted police in Wisbech after a routine check by a plumber came across a drugs factory. (Image: Cambs Police)

“Signs could include blacked out windows, people coming and going at various times of the day and noticing a distinctive ‘hum’ noise from the fans and ventilation if living nearby.

"In addition, being able to recognise the signs of modern slavery may help save someone’s life.”

He added: “"Warning signs include people seeming overly tired, withdrawn or avoiding eye contact, not having suitable equipment or clothing for the job they are doing, not being able to come and go as they wish, being unpaid or paid very little and working excessively long or unusual hours."