A woman who took offence to a passage in a book written by Val McDermid nearly 30 years ago was convicted of common assault on Tuesday for throwing ink over the writer at a book signing.

In dramatic court scenes, defendant Sandra Botham stormed out of Sunderland Magistrates Court after unleashing a tirade of insults, calling McDermid a "liar", the Sunderland Echo reports. Botham was convicted in her absence, and is yet to be sentenced.

McDermid has written 31 books including crime novels, short stories and children's books, and is much-loved by fans for her fictional crimefighters, Manchester private investigator Kate Brannigan, and the crime-busting duo of detective chief inspector Carol Jordan and clinical psychologist Tony Hill. She told the court that in 25 years as an author she'd never experienced anything like it before.

Sandra Botham, of Hendon, had harboured a grudge against the bestselling author McDermid over a paragraph in the 1985 novel A Suitable Job for a Woman, which refers to a woman called Sandra who was shaped "like a Michelin Man".

In a bizarre premeditated attack on 6 December last year, Botham dressed up in a disguise of a blonde wig and glasses and attended a lecture on crime writing by McDermid, who was promoting her book Vanishing Point. At the book signing, Botham produced a dog-eared copy of A Suitable Job for a Woman, asking for it to be dedicated to "Michelin Man San".

McDermid said: "I did, even though I had no idea what that was a reference to. She then produced a book that looked like an old Top of the Pops annual and opened it at a page with a picture of Jimmy Savile on it. She asked me to sign that too. She was quite insistent. I wasn't keen, but I get asked to sign all sorts of things so I thought it would be easier just to do it."

Botham then took out a container of ink and threw it at the author, getting it in her mouth and over her clothes.