A geologist who studied hazardous gases in Zane Gbangbola's home told an inquest the house was not safe to live in if flooding were to reoccur because the source of the gases was not clear.

Zane, seven, a pupil at St George’s Junior School in Weybridge, died after his family home in Thameside, Chertsey, was flooded on February 8, 2014.

He had been sleeping in the spare bedroom after watching television with his mother Nicole Lawler.

His family claims he was killed by hydrogen cyanide gas that leaked from a former landfill site behind their home, but this is disputed by Spelthorne Council and the Environment Agency.

Low levels of carbon monoxide were found in Zane's blood during a post-mortem examination, which concluded that this was the cause of his death.

The family had bought a number of electric pumps to remove floodwater from their home and later rented a petrol-powered one as back-up.

Gavin Roberts, a chartered geologist and technical director at Geo-Environmental, carried out studies of Zane’s home and garden on behalf of the family’s insurers to look for plausible sources of hydrogen cyanide in May 2014.

He told Woking Coroner’s Court today: “We attached hydrogen cyanide monitors that were set to give an audible alarm if the short term exposure limit was detected.

“We also undertook soil and water testing.”

Mr Roberts and other geologists working in his team detected 1 part per million of hydrogen cyanide from boreholes in the garden that reached to ground water under the home.

They also found 5 parts per million of carbon monoxide in the property – short term exposure limits are at 200 parts per million.

When asked for his conclusion, Mr Roberts said: “All the monitoring is done under non-flooding conditions, we didn’t find anything of concern.

“Going back to the risk assessment, we found no evidence of anything at the property so we looked for other sources in the groundwater flow.”

However, Mr Roberts was prevented from analysing land beyond the property, including the former landfill site nearby, as it was deemed “third party private property”.

He stressed that he would have been “trespassing” had he tried to analyse that land without permission.

But Mr Roberts said the family home was not safe to live in, particularly if flooding became a risk again, because the source of any hazardous gas “has not been categorically ruled out”.

Steven Wilson, a chartered civil engineer and chartered environmentalist of the Environmental Protection Group, was hired by Spelthorne Council to assess whether there were levels of hydrogen cyanide in Zane’s home.

He said: "It seems unlikely that there is sufficient levels of hydrogen cyanide in the property to cause Zane's death"

The inquest continues.