She was born Sheila Harrington at Griffithstown, Pontypool, on October 25 1920. Her father Frank Harrington was a local GP who had served as a medical officer in the Indian Army, retiring as a full colonel.

Her mother Molly was a devout Irish Catholic who sent her daughter, commonly known as Bunty, to La Sagesse school, run by the French order the Daughters of Wisdom at Jesmond Towers, Newcastle upon Tyne.

After leaving school, Sheila worked as a secretary at the BBC in Bristol. She joined the Auxiliary Territorial Service (ATS), the women’s branch of the Army, in December 1942 and after training was selected for what was described as a “special and possibly dangerous” assignment with the Auxiliary Units.

She was given a password and told to meet an ATS major, Beatrice Temple, niece of the then Archbishop of Canterbury, in Harrods, where she was interviewed over tea on July 21 1943 and accepted for a post with the Special Duties Branch.

After tests and training at the MI6 signals training school in Hans Place, just behind Harrods, and at the Auxiliary Units’ communications headquarters at Coleshill House (then in west Berkshire, now in Oxfordshire), she was sent to Alnwick in Northumberland.

The Auxiliary Units were split into two separate branches, Operations – 4,200 hand-picked members of the Home Guard who would form the initial resistance fighters – and the intelligence-gathering Special Duties Branch.

The War Office took charge of the Auxiliary Units in June 1940, but for security reasons MI6 retained control of its communications links, which were operated by specially trained members of the ATS.

The Special Duties intelligence agents, largely based in coastal areas that were likely to be the first to see the invading Germans, were selected from ordinary members of society – ranging from farmers, doctors, clergymen, housewives and in at least one case a pub barmaid.

They were trained by MI6 to leave their intelligence reports on the German activities in “dead letter boxes” such as hollowed-out trees, chicken coops and gaps in brick walls from where they would be collected by couriers who would take them to the local wireless operator.

He or she would then encode the reports and transmit them to their regional control station, each manned by a team of three signallers who operated from underground shelters equipped with cooking, sleeping and lavatory facilities and a month’s worth of food and fuel.

Each of the 30 control stations received the intelligence reports of between five and 10 local networks and then sent them on to the communications headquarters at Coleshill House. Most were manned by three ATS subalterns recruited by Temple.

Sheila was sent to work in a dugout set in the remains of an iron-age earthwork next to the A1 at Heiferlaw just north of Alnwick. She and her colleagues were accommodated in nearby Doxford Hall, now a hotel.

Although no invasion materialised, the networks carried out exercises and sent dummy reports. But the nearest Sheila came to enemy action was during an air raid, when she was blown off her bicycle by a bomb blast and broke her arm.

The Auxiliary Units were stood down in December 1944. After the war, she married Kennedy Trevaskis, a colonial administrator, at Brompton Oratory on August 25 1945.