'They have lied and spied for the communists': The suburban spies who sold nuclear secrets to the USSR

The Portland spy ring trial began on 13 March 1961. The BBC reported on a Cold War tale of espionage, adultery, exorcism – and a deceptively ordinary-looking bungalow.

The enduring image of a Cold War spy swap is of two freed captives walking slowly from opposite sides of a bleak checkpoint or bridge towards liberty. When Helen and Peter Kroger were released from prison in 1969 in exchange for a British lecturer held in the Soviet Union, it was a different story.

Standing beside them on a plane bound for Poland, the BBC's Tom Mangold reported: "Within less than an hour, they'll step onto the communist soil for which they have lied and spied and spent nine years in prison. After nine years: smoked salmon, egg, chicken, champagne and coffee for the Krogers as they sit, sublimely happy, in the seats originally allocated in this first-class lounge to BBC television news."

The Krogers had been convicted in 1961 of being part of the notorious five-strong Portland spy ring, one of most significant espionage operations of the era. The couple's ingenious communications set-up involved sending valuable British intelligence files to Moscow. Many felt they were getting off lightly.

The focus of the spies' attention was the Admiralty Underwater Weapons Establishment in Portland, a peninsula on the sandy southern tip of England. At this research facility, scientists and technicians worked on some of the Royal Navy's most sensitive projects, such as anti-submarine warfare systems. Despite the highly confidential nature of the work, every item being trialled was recorded meticulously. Each new device and proposed modification had its own file filled with photographs, drawings and technical specifications. To get their hands on these, the Soviets needed professional spies. Two of them were based inside the research facility itself.

Harry Houghton was a former Royal Navy officer who had been recruited by communist intelligence while working at the British Embassy in Poland. On his return to the UK in 1953 he resumed his spying activities when he got a job as a clerk in the Portland facility. He began having an extramarital affair with Ethel Gee, a colleague who had access to material of a higher classification. Posing as husband and wife, the couple would travel up to London to meet their KGB contact and hand over their stolen material.

It emerged in files released by the British intelligence service MI5 in 2019 that they could have been rumbled much earlier thanks to a tip from Houghton's actual wife, who had become suspicious about his regular London visits. Mrs Houghton said that upon his return from one trip, he "pulled out a bundle of pound notes and threw them in the air". On another occasion, she opened a parcel on his desk and found a bundle of papers marked "top secret". And when she asked her husband about a tiny camera she had discovered hidden under the stairs, he became angry. But her allegations had been ignored as it was felt that they were "made on the spur of the moment and out of pure spite".

The tiny camera spotted by Houghton's wife was a key weapon in the spies' arsenal. Photographs taken with microdot cameras could reduce whole pages of information so that they fit onto a single tiny piece of film. This could be embedded onto paper in an area the size of the full stop at the end of this sentence. One minuscule dot could contain detailed photographs and sketches. Concealed on a postcard or inside a book, these secret messages........

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