I wonder if the Bank of England knew what they were letting themselves in for when they agreed to put Jane Austen on the £10 note. Janeites have been arguing over the authenticity of portraits for decades. The most settled on is the watercolour sketch held by the National Portrait Gallery and attributed to Austen’s sister, Cassandra. It was offered to James Edward Austen-Leigh (their nephew) by one of their great-nieces for inclusion in his 1869 Memoir.

Not thinking her pursed lips and crossed arms proper enough for the frontispiece, Austen-Leigh commissioned a new version, which softened her expression – and her features – and put her hands neatly on her lap. The artist, John Andrews, took a few liberties with her figure and dress too. Her shoulders are less broad, her waist tucked in and her chest shallowed so that her head looks enormous. He filled in the frills and ruffles that Cassandra hadn’t included: maybe Austen didn’t want to sit around all day having her picture done.

The image that will appear on the banknotes, though, is neither of these but an engraving made in 1870 from Andrews’s painting. The engraver made things still worse and the now most widely reproduced picture of Austen is of a diminutive, large-eyed, round-faced girl with trembling lips (and a very awkward figure), not the cross and impatient-looking woman of Cassandra’s sketch. One of her relatives said: ‘it is a very pleasing, sweet face, -tho’, I confess, to not thinking it much like the original;- but that, the public will not be able to detect.’

But as Claudia Johnson points out in Jane Austen’s Cults and Cultures, there’s no record of Cassandra’s sketch before 1869. She painted many watercolour miniatures, but this one isn’t mentioned in her will or letters, or in the list she made of Austenian keepsakes. The National Portrait Gallery decided to accept it as authentic – as many Janeites have – out of custom rather than evidence. The only undisputed image of Austen we have is another of Cassandra’s watercolours. Stolen hastily while she sat outside with her bonnet untied, it shows Austen with her back to us, her face obstinately hidden. I think that’s the image I’d like to see on the £10 note.