The couple believe the inquest into their son's death, which concluded in September 2016, was flawed. Zane's death was attributed by the coroner to carbon monoxide poisoning from a petrol-powered pump used by the couple to clear floodwater - but which they insist was not in use.
They believe a former landfill site behind their home in Chertsey was the source of hydrogen cyanide gas, which was forced up and out of the earth by the "piston effect" of the swollen river raising the level of the water table underground, and it was this gas that killed Zane.
Firefighters found high levels of hydrogen cyanide at the property, according to the Fire Brigades Union (FBU), which has "serious concerns" about how the inquest was conducted.
They have spent the past six years gathering information they say was not explored or disclosed at the inquest, and which they want to be put before an independent panel inquiry.
This includes the existence of a Defence Evaluation and Research Agency (DERA) military tank-testing facility, identified as a former industrial site in Runnymede council's land contamination strategy.
A former Ministry of Defence (MoD) engineer, who asked to remain anonymous, has told the BBC he believes subcontractors working for the tank-research facility five miles from the house used to dump waste chemicals in local gravel pits, including behind Zane's house.
He said those chemicals were by-products from anti-rust and anti-corrosion tank coatings and he claimed they could produce cyanide. They would have been put into drums that rusted and corroded over the years, allowing chemicals to leach into the ground, he alleged.
The BBC has agreed to protect his identity as a former MoD worker, but has therefore been unable to verify the claims.
It recently emerged during a commemoration for Zane, held at the House of Lords, that experts from the government's defence laboratory, Porton Down, were involved in the response to his death.
At that event, Manchester's Labour mayor Andy Burnham said: "I want the truth about Zane. I don't think we do have the truth about what happened to this lovely little boy.
"There were many authorities involved," the former health secretary said. "Porton Down was also involved. Did any of this come out in the inquest? No it didn't. Therefore, the full story clearly wasn't told at the inquest and it needs to be."
In his report at the conclusion of the inquest, the coroner Richard Travers cited an expert who said high levels of hydrogen cyanide were rare and all associated with former gasworks, while a council officer said there had been no significant pollution incidents on the land behind Zane's home since the 1950s.
However, another report referred to by the coroner did acknowledge that illegal tipping could have taken place, and a further expert, geo-environmentalist Gavin Roberts, told the inquest it was possible floodwater had triggered a one-off discharge of hydrogen cyanide and carried it to the basement of the house.
The coroners report did not look at the area's military and defence history.
In December, the FBU's general secretary Matt Wrack wrote to the family: "Our members attended that fateful day in 2014 and found hydrogen cyanide in your home."
He said there were "disturbing gaps in the evidence", adding: "The unregulated landfill site adjacent to your home seems to have released the nerve gas and there must now be an independent inquiry so this can be looked at, and those found responsible must be held to account."
The BBC has taken all claims about historical contamination in the area to the Ministry of Defence (MoD), the Environment Agency and Defra, and also to the two local authorities, Runnymede and Spelthorne councils. The tank facility was run by the former Defence Evaluation and Research Agency, which was part of the MoD and was later split into the Defence Science and Technology Laboratory (DSTL) and Qinetiq. The BBC approached DSTL, which has its headquarters at Porton Down, for comment but was referred to the MoD.
- The MoD declined to comment unless formal allegations were made.
- Defra, commenting for the government and the Environment Agency, said: "This is a tragic case and our thoughts remain with the Gbangbola family. There has been an inquest into the cause of Zane's death. The inquest heard from many witnesses, and throughout the Environment Agency provided detailed evidence to assist the independent coroner in reaching his conclusion."
- Runnymede council said to the best of its knowledge it had no information regarding potential contamination from the former DERA operation. It said the site was identified as an area of potential contamination due to its former use but the ongoing redevelopment of the land had been through the planning process and conditions were put in place requiring contaminated land investigations and remediation.
- Spelthorne council said the coroner's report set out details of tests on the land carried out by the landowner, the Environment Agency and the family's insurers and quoted the coroner's statement that "on the balance of probabilities...there was no hydrogen cyanide present in the house". It said the lake behind the house was analysed quarterly by the landowners and was a healthy environment, adding that the landowner had analysed an on-site borehole for ground gas on a regular basis for a number of years.
- Asked if there was any evidence of illegal dumping by DERA subcontractors, Spelthorne council said "no".