During this time she worked with John Caldwell, specialist in early keyboard music, and with Christopher Page, who advised her DPhil thesis, Iconographical Evidence for the Late-Medieval Organ (1986; published by Garland in 1989).
She is known as an expert on late-medieval organ music and was invited to contribute entries for the Oxford Dictionary of the Middle Ages (2010) as well as to present the keynote lecture for the inauguration of the medieval Blockwerk in Amsterdam's Orgelpark (2013).
In 1985, Marshall was awarded first prize at the St. Albans Organ Playing Competition, earning her a recording contract with the BBC and a recital in London's Royal Festival Hall.
During her tenure there from 1993 until 1996, she set up and implemented a Master of Music degree between the Academy and King's College London to enable postgraduate musicians to pursue academic research alongside their performance studies.
Marshall's compact disc recordings feature music of the Italian and Spanish Renaissance, French Classical and Romantic periods, and works by Johann Sebastian Bach.
She has also made a recording of works for organ by female composers, Divine Euterpe,[3] that includes music by Fanny Mendelssohn Hensel, Elfrida Andrée, and Ethel Smyth.
Marshall is the advisor on organs for the Musical Instrument Museum (MIM)[4] in Phoenix and has made videos for their exhibits in Guanajuato (Mexico), Toulouse (France) and Florence (Italy).