When Leading Aircraftman Ronald Maddison walked into the sealed gas chamber at Porton Down, his family believe he thought he was taking part in an experiment to cure the common cold.

It was 1953, and the 20-year-old had been tempted by an advert promising volunteers for tests at the government’s secretive chemical laboratory 15 shillings and an assurance no harm would come to them.

LAC Maddison was planning to use the small fee to buy an engagement ring for his girlfriend, Mary Pyle. An hour later he died in agony.

Drops of the nerve agent Sarin had been dripped onto his arm through two layers of cloth but had absorbed through his skin much faster than the scientists had expected.

His sudden death marks one of the darkest chapters in the history of the chemical facility, which identified the nerve agent used in the attack on the on former Russian spy Sergei Skripal and in the mysterious case of Charles Rowley and his girlfriend, Dawn Sturgess, in Amesbury.