Colleagues must have wondered why Brian Trevelyan was suddenly so quick to leave the office at night.
If they had asked, he could have told them he was rushing home to help his partner with some market research without them raising so much as an eyebrow.
It was the truth – although the market research was raising rather more than that...
His girlfriend was engrossed in writing an erotic novel. And she had decided there was no better way to judge if it was sexy enough than to use Brian as the, er, yardstick.
He was her human guinea pig every night for three months. And funnily enough, he never complained once.
Jude Calvert-Toulmin giggles at the memory: "I wrote a chapter every day and as soon as Brian got in from work, I'd strip him naked, sit him on the bed and make him read it.
"I would sit opposite with a clipboard and pen and 'monitor' any reaction. It was my first attempt at erotic writing; I had to know I was on the right lines," she says.
Was she? "I must have been; it always worked," she smiles. "It would invariably end up with me throwing the clipboard to the floor and a hot sex session taking place!"
Brian, a health and safety officer at Sheffield Hallam University, helped out in other ways, too. A writer in his spare time, he edited her proofs. And it goes without saying that her lover of seven years did provide much of the inspiration for the steamy sex scenes.
But that is where real life ends and fantasy begins, stresses Jude. The 49-year-old mother of three and grandmother of one does not want anyone to get the wrong idea about her book.
You see, it is called Mother-in Law, Son-In-Law. And the theme is an affair that develops between a middle-aged woman and her daughter's husband.
It's a taboo subject. And not one that Jude herself had ever contemplated.
"I do have a son-in-law of sorts; he is my grandson's father, though he and my daughter are no longer together. He's a lovely lad and I have never, ever had any lustful thoughts about him," she says emphatically.
She realised that other women DO fancy their daughters' partners quite by chance.
The archetypal voluptuous blonde from Norton Lees is also a photo-journalist for both the climbing and the music press (back in the Eighties she photographed Bob Geldof for Melody Maker).
She set up an innocent internet blog with the sole intention of publicising her work – and to her horror discovered that thanks to the way search engines work, her site was being accessed by mistake by kinky types looking for stories about mothers-in-law having affairs with their sons-in-law.
"I was slightly appalled when my web-stats showed around 10 people a day were arriving on my blog because they had typed in Mother-in law son-in-law sex," she says.
" I couldn't understand why. The only mention of my son-in-law referred to the time we had taken him to a climbing wall and discovered he was a natural. The sex word was elsewhere, in unrelated articles.
And yet my blog came up third on Google!"
Suddenly curious, Jude did some searching around and found a U.S forum with a thread on the subject.
"It had been running for the last three years. I was amazed. Men were talking about their pent-up lust for their partner's mother. Mature women were fantasising about their daughter's partner. It was clear there was an interest in the subject, so I decided to write a novel about it."
It's an unusual way to find inspiration, Jude agrees. And an unusual topic to choose.
"It's not something I would ever contemplate. I think there are moral boundaries that shouldn't be crossed. People could be badly hurt by the breach of trust that such a relationship would involve," she says.
"But human beings are complicated things and we should try to understand the diversity of human desire without condemning," she says. "It is a known fact that younger men find older women attractive.
"Most men I know say they used to have thoughts about their friends' mothers when they were young. And maybe for some adult men, the mother-in-law is the more nurturing, caring version of their wife and that sparks some sort of attraction."
She is keen to stress, too, that in her self-published (Fleur De Lys Ltd) book, there is a moral undertone.
"The protagonist of the novel, Julia, is a 55-year-old widow. Her daughter Kate is already in the throws of an affair and doesn't appreciate her dependable, solid husband Alex. He and Julia, who have both harboured private fantasies about one another for a long time, are thrown together by Kate's selfishness and passion ensues.
"And despite the explicit sex, this is still essentially a love story," insists Jude. "I wrote it partly to show people having such thoughts that they are not the only ones."
Since being launched on Amazon.co.uk just before Christmas, the book has gone straight in at Number 16,000 – not bad when you consider there are 3.5 million books in the online shop.
There has been an amazing side-effect as well...
It's going down a storm with married couples – as a sex aid.
"Couples have posted reviews saying it has spiced up their sex life, which I'm delighted about," she beams.
One woman has written: "My husband, it has to be said, was happy with my purchase, as I dragged him to bed after reading sections on more than one occasion!" and another called it "The literary version of Viagra."
For those contemplating writing a racy novel of their own, Jude advises: "You have to write honestly, put yourself into the situation and describe it without making it sound like you're trying too hard."
Didn't she find herself blushing as she typed?
"No, the embarrassing bit was when relatives insisted on reading it. Fortunately my son-in-law no longer lives in Sheffield and knows nothing about it, but my daughters insisted on seeing it."
"They summed it up in just one word: "Gross".
Though they hope it makes me lots of money so I can buy them things!"
Worse, still, though, was when she told her mother about it.
"I said, mother, you're the only person who isn't allowed to read it. She demanded I post it to her and then I got a letter from her.
"I was trembling as I opened it but she was writing to tell me that she was so proud of me and loved it so much she was going to pass it round all her friends. She's 75!"
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