The explosion at Abbeystead killed eight people immediately, with another eight dying later of their injuries.
Nobody who was in the underground valve house escaped unhurt.
The mystery surrounding the cause of the explosion continued for some time, as tests in the immediate aftermath revealed no trace of gas, nor were there any gas installations in the pumping station itself.
However, an investigation by the Health and Safety Executive revealed that the siting of the Abbeystead pumping station underground, and close to seams of coal, had made it vulnerable to build-ups of methane gas estimated to be 20,000 years old. The station had also been unused for several weeks before the visit, and the gas had accumulated in the water pipe leading into the valve chamber.
It was then pumped into the valve house during the demonstration, creating a lethal inflammable atmosphere.
The investigators never found out how the gas came to explode. However, nobody involved in building the pumping station had realised that gas could be a problem, so both guests and workers were allowed to smoke.
The Abbeystead pumping station was refurbished and returned to use following the accident.
In March 1987, at Lancaster High Court, a judge ruled that the designers, Binnie and Partners should bare the brunt of the blame – 55 per cent – for being negligent in failing to exercise “reasonable care” in assessing the risk of methane.
The North West Water Authority (NWWA) was found to be 30 per cent to blame for failing to ensure the plant was safe for visitors and employees by testing for methane.
And the plant’s builders, Nuttalls Ltd, shared 15 per cent of the blame for negligence in failing to carry out systematic tests for methane.
But the legal battle did not end there. All three companies appealed. While Nuttalls and the NWWA were cleared Binnie and Partners was not, leaving the company to shoulder the blame. The company again tried to appeal but were denied by Law Lords. Eventually the company came to an out of court settlement with a consortium of 26 for £2.5m.
Thirty-six of the 44 visitors to visit the Abbeystead plant that night were from St Michael’s.
Among the dead were three of the NWWA officials who were acting as guides.
The victims were: James Rowland Birtwistle, Frank Coupe, Mark Edmund Eckersley, Pauline Elizabeth Eckersley, Herbert Charles Gardner, George Alan Lacey, William Mason, William James McGarry, John William Myerscough, Ralph Trevor Rawlings, Geoffrey Seed, Geoffrey Standing, Albert Tomlinson, Edna Tomlinson, Edith Freer Tyson and Penelope Ann Weild.