With thanks to Chris Dubbs for finding the photograph that sent me off on this voyage of discovery and to Lynne Sidaway for additional information. It is proving very difficult to find information about Winifred.
Born Winifred Sayer in 1884 in Bacton, near Yarmouth in Norfolk, UK, her parents were William Sayer, a solicitor, and his wife, Ettie Sayer. Winifred became fascinated by all things mechanical and learnt to drive in her teens, quickly becoming an expert motorist.
In 1904, Winifred married George Cecil Buller, a businessman, who was Managing Director of the Shoreham and Lancing Land Company. The couple had 2 children – Max (Donald) Napier Buller (1907 – 1993) and Cecil Edward Anthony Buller (1906 – 1972). In the 1911 Census, George and Winifred Mea Buller were listed as living at 60 Draycott Place, Chelsea, London. It seems they also had property in Shoreham-by-Sea, Sussex, for their sons were there according to the 1911 Census, in charge of a nanny.
Winifred met the French aviator Count Olivier de Montalent when he was in Britain in 1911 with his Breguet plane. She took several flights with him, which made her decide to learn to fly. When her husband was away on an extended business trip, Winifred drove her sons and their nanny to France so that she could take flying lessons at the aerodrome in Douai. La Brayelle Airfield was one of the first airfields in France. It was situated 3 kilometres (1.9 mi) west of Douai, in the Nord département in northern France. It was host to the world’s first aviation meeting, home to Bréguet Aviation and an important airfield in the First World War. 
Winifred soon became an accomplished and skilled pilot and on 18th May 1912, the request of the Aero Club de France for permission to issue an Aviator's Certificate to Mrs. W. Buller, a British subject, was granted. Back in Britain, Winifred became the English ladies cross-country flying champion in 1912, holding the record for long-distance and cross-country flying. . She then became the first woman to take aviation seriously enough to adopt it as a profession and worked as a test pilot for the British Caudron Company, based in Cricklewood, near Hendon Aerodrome, north of London.
A report in the “Broughty Ferry Guide and Advertiser”newspaper of 3rd July 1914 announced that Winifred joined the flying corps of the British League in April 1914, which was intended for service in Ulster in case of military opertions.
During the First World War, Winifred joined the American Volunteer Motor Ambulance Corps and, as this photograph shows, served on the Western Front.
I have not been able to verify further details about Winifred. It seems she may have gone to America with her husband and sons. Several websites mention that George Buller became a naturalised US citizen in 1927 and that Winifred died in Hove in 1970, by which time she was Mea Winifred Williams. However, I found several women with similar names, so it is difficult to be certain. If anyone has any definite information about Winifred please get in touch.
WW1 Photograph of Winifred in her plane on the Western Front – Original caption: "Red Cross Nurse on Battlefield In Aeroplane. Mrs. Winifred Buller, the English airwoman, who is now on active service with the Volunteer Motor Ambulance Corps, is ready to fly over the battlefield with her wounded charge should the ambulance in which she is conveying him break down. The photo shows Mrs. Buller in her aeroplane. (Photo by George Rinhart / Corbis via Getty Images)"
Sources: Find my Past, Free BMD, British Newspaper Archive,
Chronicling America and https://www.gracesguide.co.uk/1912_Aviators_Certificates_-_UK
