There could be no better spokesperson for Fanny today than her three-times-great-granddaughter, the filmmaker Sheila Hayman. Among her documentaries are Mendelssohn, The Nazis and Me, and more recently Fanny: The Other Mendelssohn, tracing the verification of Fanny’s authorship of her Easter Sonata for solo piano, a work hitherto not only little known, but assumed to be by Felix.

Hayman points out that the private nature of Fanny’s composing brought some perhaps unexpected artistic benefits. ‘While Felix had to please everybody with very public compositions, Fanny, needing to please only herself, could write literally whatever she liked,’ she says.

For orchestra, Fanny composed only a single Overture in C, which dates from 1830-32 and was designed for performance in the family’s glittering salon. Two years after its premiere, Fanny conducted a second performance. As ever, she evinced modesty: “Had I not been so shy, and embarrassed with every stroke, I would have been able to conduct reasonably well,” she told Felix.

Although the overture is relatively conventional, her String Quartet of 1834 pushed the boat out, embracing the potential of romanticism to an impressive degree. Felix disapproved of the work – which was only performed once in Fanny’s lifetime – regarding its adventurous nature as undisciplined. But the lyricism and subtle darkness in the Romanze, its third movement, occupies a sphere that is wholly and unmistakably Fanny’s personal world.

Fanny found happiness in family life with Wilhelm Hensel, court painter to King Friedrich Wilhelm III. The couple were deeply and devotedly in love, but again her father threw a spanner in the works, making them wait seven years before marrying. Fanny, though, was no stranger to determination.

Wilhelm proved refreshingly supportive to her; he even provided her each morning with a fresh piece of blank manuscript paper to fill. Soon the couple had a son, Sebastian. But there were travails and tragedies in store: Fanny suffered at least one miscarriage and had a daughter who was stillborn. She also had to nurse her family through a cholera epidemic. Despite everything, she had music in her life every day. In her Berlin home, she rehearsed and conducted her own choir and put on weekly concerts in her salon.

She grew, meanwhile, into a powerful, uncompromising personality. ‘Sebastian Hensel, her son, wrote a magisterial tome about the family Mendelssohn, from Moses Mendelssohn [the composers’ grandfather, the Enlightenment philosopher] until the year of Fanny and Felix’s death,’ Hayman recounts. ‘It’s a fantastic pen portrait of her character and her absolute zeal.

‘She was somebody completely incapable of disguising her feelings. If she liked people, she loved them; if she didn’t like them, you saw it all over her face. She hated parties, formal gatherings of any kind, paying calls, giving cards and wearing hats because the veils got caught in her glasses. And she couldn’t see the point of jewellery.’ Once, she recounts, Wilhelm was obliged to give a painting to Queen Victoria. The monarch rewarded him with an enormous emerald ring. Back from London, he presented it to his wife. Her response? ‘I can’t play the piano in that!’

Fanny comes across as affectionate, clever, sharp-witted and sometimes scary. ‘Everybody was frightened of her,’ Hayman says. The composer William Sterndale Bennett was a case in point: ‘He said that he had played for Felix often, but that having to play for Mrs. Hensel, and to think that this formidable person should be a woman, was absolutely terrifying.

‘I was attracted to Fanny’s balancing act between the urge to express yourself and to be the person that you feel that you should be, with a massive creative drive and artistic gift; but then, on the other hand, not wanting to be difficult or pushy. Fanny wanted to be a good wife, mother, sister and housekeeper, to do all the things that the family and society expected her to do. I think she found it difficult to achieve that balance. She struggled with it all her life.’

It seems that Wilhelm’s support had its limits. ‘He understood instinctively what her drives and her needs were, and absolutely supported her. But he had a cloth-ear for music. He couldn’t hold a tune. It wasn’t his thing at all, so he wasn’t able to give Fanny the affirmation that she needed from another musician.’ This was even more vital thanks to Fanny’s intense critical facility: ‘Up until late in her life, the only living musician whom she rated was her brother. She met Paganini, Chopin and Berlioz and she didn’t think much of them at all!’

There was one other person she liked and felt supported by: the French composer Charles Gounod, whom she met in Rome in 1839. ‘Gounod was in residence at the Academie Française. She decided that she could really respect him – and he was absolutely blown away by her. It seems that German music was not familiar in Italy at that point, or in France. According to what I’ve been told, Fanny brought Beethoven and Bach to Rome. She sat down and played five Beethoven sonatas in a row, to show Gounod what he’d been missing. Therefore she was admired and esteemed as much for the musical knowledge and tradition that she brought with her as for her own extraordinary gifts.

‘That encouragement from Gounod helped to precipitate Das Jahr – probably because in that year in Rome, as she said herself, “I was never made so much of in my life, and it’s really rather pleasant!”

‘Your creativity blossoms when you feel confident in what you’re doing and other people are confident in it as well. After a whole year of that, away from domestic responsibilities, she came back full of ideas and full of confidence that they were worth putting down. She wrote Das Jahr, but didn’t publish it, and mentioned it only in one letter. That’s what’s so extraordinary: she writes an absolutely barnstorming piece of music, yet refers to it in a casual way as a little bit of fun with which she’s been amusing herself and that might perhaps entertain other people.’

Das Jahr is a cycle of 12 piano pieces, one for each month. It served, too, as a love letter. It was Fanny’s present to her husband at Christmas 1841; she created a beautiful manuscript of it with space at the head of each movement for him to add his drawings. She also appended an extract of poetry to each piece, selected from Goethe, Uhland, Schiller and Tieck.

January is headed ‘A Dream’, illustrated with a portrait of Fanny, whom Wilhelm seems to portray (fittingly enough) as St Cecilia, the patron saint of music. February is for Carnival, an Italianate tarantella preceded by lines from Goethe’s Faust. March, with another extract from the play, leans towards Easter with a quotation of the Lutheran chorale Christ ist erstanden. There is a light-hearted family scene for the spring sunshine of April, while May is entitled ‘Fruhlingslied’ – Spring Song, a heading used more than once by Fanny’s brother. June’s ‘Serenade’, is a deeply Mendelssohnian song without words, and July an evocation of summer heat, quoting from Schiller’s ‘Der Abend’, before August presents a harvest parade.

September, ‘The River’, draws upon Goethe’s melancholy poem ‘An dem Mond’ (To the Moon); in October, the huntsman’s horn sounds through the green forest. November is a darker piece, hinting at the feast of All Soul’s and a song of sorrow as winter approaches. Finally there is Christmas magic in December, with flurries of snow, a celebratory chorale and Wilhelm’s portrait of a winged Fanny – an angel, attended by a cherub, probably little Sebastian. Not all the drawings are so exalted; one shows the first German train, on which Fanny and Wilhelm had travelled with much excitement between Berlin and Potsdam.

Das Jahr has been recorded several times, but scarcely often enough to have emerged from its status as buried treasure. Hayman thinks that it remains neglected because it is so difficult to play. Besides, it has been under the radar for the same reason that Fanny’s other works have taken so long to come to light.

After a rehearsal in May 1847, Fanny was taken ill; hours later, she was dead, having suffered a stroke. Felix, devastated, died of the same condition that November. ‘When all the family papers were divided up,’ Hayman says, ‘everybody wanted Felix’s because they were considered valuable. Nobody wanted any of Fanny’s; they were kept only for sentimental reasons. The new critical edition of Fanny’s Easter Sonata is the first time a work of hers has been put out by a major publisher.’ She had left well over 400 pieces of music.

‘Even women who were celebrated as composers in their lifetimes did not join the musical canon after their deaths in the same way as their male colleagues,’ Hayman says. But for Fanny, it is not too late, even if it is later than her music deserves. With Hayman’s film and now the OAE’s Das Jahr project in the ascendency, this indomitable figure is gaining the limelight at long last. And she might indeed have found it ‘really rather pleasant’.