Her neighbours knew she was a life-long Communist who still took the Morning Star - she would buy 32 copies of each issue and hand them out to friends - but she never appeared anything other than harmless, the only evidence of radicalism were the CND posters in the bay window of her semi.
Now she is unmasked as the agent who leaked key Western secrets to the Soviet Union for more than 40 years, only stopping when she retired in 1972: secrets that changed the history of the 20th century.
During the Second World War she gave vital intelligence of British and American development of the atomic bomb to her KGB handlers. This helped Stalin to manufacture his own bomb two years ahead of schedule.
Her treachery would have remained a secret had Russian intelligence archivist Vasili Mitrokhin not defected to MI6 in 1992. He brought with him a treasure trove - six large trunkfuls of KGB files. One of the agent files was marked with the codename "Hola" and revealed the existence of a a top agent who had remained undetected for more than four decades.
Melita Norwood, nee Sirnis, was born in 1912, the English daughter of a Latvian father and a British mother, and grew up in unremarkable circumstances in the Home Counties. At the age of 20, in 1932, she joined the innocuous sounding British Non-Ferrous Metals Research Association in London as a clerk. In the early 1930s she also joined the Communist Party.
In 1935 Miss Sirnis was recommended to the NKVD, a predecessor of the KGB, by Andrew Rothstein, one of the founders of the British Communist Party. As the Spanish Civil War - the first great struggle between Communism and Fascism - began, she was recruited.By 1937 she was a full agent.
In 1930s the Soviet Communists were desperately trying to turn a feudal, agriculture-based nation into an industrial society capable of mass-producing modern armaments. Key to this was research into metallurgy and this is exactly what Melita Norwood had access to in her apparently lowly job.
As early as 1938, she was nearly caught by the British. She had links with a spy ring working for the Soviets in the Woolwich Arsenal, and three other agents were caught by MI5 and imprisoned. But they could not identify agent Hola.
Her importance can be seen in that, when purges in the foreign intelligence services prevented the Soviets from maintaining contact with many of their overseas agents, the Soviets valued her so highly they maintained contact with her and not Kim Philby. In the meantime, she married a Communist maths teacher. They bought the home in Bexleyheath, and began a family.
There was to be an unexpected and enormous bonus for the Soviets in having a spy in the Non-Ferrous Metals Research Association during the Second World War. In the early years of the war a top secret project known as "Tube Alloys" was launched, to build an atomic bomb. Understanding the properties of certain metals like uranium was a key requirement. Much of the research passed over the desk of Miss Sirnis's boss.
Quietly and efficiently she removed Tube Alloys files from her boss's safe, photographed their contents and passed them on to her Soviet controller.
In theory, the secrecy around Tube Alloys was extraordinarily tight. Prime Ministers did not tell their Cabinets. Post-war PM Clement Attlee did not tell his Cabinet, later saying that "some of them were not fit to be trusted with secrets of this kind". But courtesy of Miss Sirnis and other spies in both Britain and America, Stalin already knew.
The files show that, after the end of the Second World War, Mrs Norwood remained such an important agent the KGB and Soviet Military Intelligence fought for control over her. The KGB gained full control in 1952.
Mrs Norwood kept spying right up to her retirement from the Association in 1972. In 1958 she was awarded the Order of the Red Banner for services to the Soviet Union. When she retired the KGB gave her a pension of pounds 20 a month. In retirement, Mrs Norwood says she voted Labour and was active in CND. In her front window is a "Stop Trident" sticker.
She would probably never have been unmasked if Vasili Mitrokhin had not defected. Following the collapse of the Soviet Union, many former KGB and other Soviet spy officials tried to defect. Relatively few were taken on by MI6 or the CIA. The cost of relocating and giving a new identity to defectors was enormous and was only encouraged if the former spy had something special to offer. Mitrokhin did: his six trunk-loads of files from the KGB's most secret archive.
Next, enter Christopher Andrew, a Cambridge professor and the country's leading intelligence historian. Through his close contacts with British Intelligence he was introduced to Mitrokhin, now rehoused in a secret location in Britain.
Very unusually, MI5 allowed Professor Andrew access to the archive of Mitrokhin. Reading through the Mitrokhin files, Professor Andrew realised the importance of the British spy codenamed Hola. The file described a woman who was a "committed, reliable and disciplined agent, striving to be of the utmost assistance".
When Hola was identified as Mrs Norwood, reporters tracked her down and went to visit her at her home in Bexleyheath in Kent. Somewhat to their surprise, they discovered a slightly eccentric woman who was unapologetic about her spying. "I did not want money. It was not that side I was interested in. I wanted Russia to be on equal footing with the West."
Remarkably, the reporters' visit marked the first time that Mrs Norwood knew she had been unmasked. MI5 had never interviewed her. It seems that, although MI5 had access to Mitrokhin's files in 1992, the question of the prosecution of unmasked spies did not reach the Home Secretary until 1997.
The file on Mrs Norwood was passed to the Crown Prosecution Service. Its report was considered by the Director of Public Prosecutions and the Attorney General. But as it was thought an MI5 interrogation might prejudice a prosecution, Mrs Norwood was left to herself.
It is has been difficult to prosecute Soviet agents in the past. When Sir Anthony Blunt was questioned, he only confessed after being given immunity from prosecution. The Home Secretary and the Government Law Officers decided it would not be in the public interest to prosecute Mrs Norwood.
Professor Andrew says that he feels confused by Mrs Norwood. On the one hand, as a historian, he can understand why she was drawn to Communism in the 1930s. But, he says, she "believed in a Soviet Union that never existed". What baffles him is how she carried on when Stalin was seen to run "one of the vilest societies the world has ever seen".
Mrs Norwood still has not really answered this question. In her rather quaint way, she says, "I still do not think of myself as a spy. Other people will have to judge."