Pperhaps because she is married to Blenheim Palace archivist John Forster, or perhaps because she likes to see unfair treatment corrected — whatever the reason, retired teacher Margaret Forster felt so strongly that Winston Churchill’s grandmother had been wronged by history that she felt compelled to write a book about her.

She said: “History has been unkind to her. One historian even called her an Old Crone and I simply thought that she should be exposed for what she was: a brilliant woman with a compassionate nature.”

Given Mrs Forster’s privileged access to Blenheim archives, there may be more books in the offing about past Palace inhabitants who have not received, in her opinion, correct marks for lives well (or badly) spent.

Her book, Churchill’s Grandmama: Frances, 7th Duchess of Marlborough (The History Press) tackles well the problem of how to take a sensitive look at rich, grand people when peering back at them from the vantage point of the present age.

Too many historians, she maintains, have failed to give proper weight to the good done by Frances and her husband John Winston, the 7th Duke, in the paternalistic age they lived in. For instance, she is mildly hurt (on Frances’s behalf) by historian A. L. Rowse’s description of the Duke as “a complete full-blown Victorian prig” — a comment which, she says, “has unfairly and inaccurately coloured the image of John Winston”.

She said: “The writer makes the mistake of judging some of the Duke’s views on aspects of Victorian belief and behaviour by today’s more accommodating standards.”

So what were those good works which Mrs Forster maintains that the Duchess, mother of Winston Churchill’s father Lord Randolph Churchill, performed?

Mrs Forster said. “As the wife of the Viceroy of Ireland, she mobilised her influence fast, with the help of the editor of The Times, when in 1879 it looked as though the potato crop would again fail and bring starvation, as it had in the 1840s and 50s.”

She added: “She saw the danger instantly and, in contrast to the inefficiency with which the earlier situation had been met, she took immediate and effective action. She established her Famine Relief Fund.

“This potential catastrophe provided the very occasion that allowed Frances to reveal her worth on an international level.”

Nearer to home, the Duchess in 1858 laid the foundation stone for Bladon Primary School, still called the Duchess of Marlborough School, just a few yards away from the grave of her grandson Sir Winston Churchill. However, the rules of the school reflected, in Mrs Forster’s opinion, the ideas of the Duke rather than the Duchess when they stated in high Victorian tones that: “The school should not be altogether free, because people always value most that for which they had to pay something.”

Mrs Forster said: “Her instinct would have been to react to need.”

She believes it has been too much forgotten to what extent Sir Winston was brought up by his grandparents. Winston’s father Lord Randolph and his American wife, Jennie Jerome, took little notice of their son — being always in a social whirl — and Mrs Forster believes Frances provided the love and support not forthcoming from Winston’s parents. They would not visit him at his horrible prep school and he even had to remind them to fetch him for Christmas.

The present Duke, in the foreword to the book writes: “I welcome this publication with its vivid and stimulating insights into a hitherto overlooked member of my family.”

Certainly, the book will provoke discussion about how Frances was remembered by the Churchill family. The 9th Duchess, Consuelo Vanderbilt, wrote one or two things in her book The Glitter and the Gold, which were less than kind. For instance, she wrote that Frances received her, aged 19, like “a deposed sovereign greeting her successor” and told her her first duty was to produce an heir in order to stop Winston, “that little upstart”, from becoming Duke. This seems inexplicable, since Frances had always been kind to Winston. Mrs Forster said: “Consuelo’s memory for events that occurred more than 50 years before may not have been perfect.”

The author has lived in Oxfordshire for more than 20 years, having moved when her husband came to work at Blenheim. “I had little difficulty in getting a job at the Blessed George Napier School in Banbury, which I loved; it is such a happy school. Now, since retiring I love writing about Blenheim people. It draws you out — and there are half a dozen more I would like to tackle.”

She will talk at the Woodstock Literary Festival on September 17.