The fictional university city called Salster, portrayed in Alis Hawkins's novel Testament, bears more than a passing resemblance to Oxford. But the author says that Kineton and Dacre, the 14th-century college on which it centres, is not based on any specific place and in a way "its openness is the antithesis of an Oxford college".

The main college building's octagonal shape and beautiful wood and glass roof were inspired by the Lantern Tower of Ely Cathedral. The ambitious vision of medieval architects and craftspeople and the scale of what they achieved using the most rudimentary technologies still provokes awe, Alis says: "You think: People-power built this'."

The historical mystery story begins when a medieval wall painting is uncovered after a fire. The narrative darts backwards and forwards between the present day, when the college is facing an identity and fiscal crisis, and 600 years ago, when it was being built by a wealthy vintner whose radical ideas about education challenged the authority of the Church.

The true purpose of education and several other themes are explored, not only through the characters, but by the use of metaphors based on the college buildings themselves: as they are now; weathered, ancient and crumbling, succumbing to the depredations of nature and graffiti-spraying vandals - and as they were when they were first built, their honey-coloured ashlar-facing smooth and unmarked. The book evokes the original masons' and carpenters' empathy with their materials; their profound knowledge of how stone and wood look, feel and behave.

Alis's interest in medieval architecture dates from 1981, when she came to study English at Corpus Christi College. "There were no medieval buildings where I grew up, in west Wales. Coming to Oxford was just astonishing; there were all these incredible buildings. I remember eating breakfast on my first morning, thinking I'm going to eat in this 16th-century hall every day!'"

She then trained as a speech and language therapist, but continued to develop her interest: "I began looking at medieval buildings in a more informed way, reading as much as I could get my hands on through inter-library loan. I'd just follow the bibliography, then the next one."

She discovered that at that period it had been possible for women to work in craft professions such as stonemasonry and carpentry, and Gwyneth of Kineton, one of the central characters in Testament, is a female master carpenter. "In the high medieval period, women had far more autonomy and independence than later on."

Another theme of the book, the way that parents and societies react to people with disabilities, grew out of Alis's day job. Now she works with young people with autistic spectrum disorder, but she previously taught children with other special needs. Gwyneth's son Tobias has what would be diagnosed today as cerebral palsy. His brave struggles to communicate are clearly informed by Alis's first-hand experience of children with language difficulties.

Testament is the fourth book Alis has written, but the first to be published. She first had the idea for it 17 years ago. "I knew it was a good idea - and there was no way I was going to waste it on my first book." She wrote other books for practice, until she felt ready to embark on it. Even then, she rewrote the 21st-century strand four times, finding it easier to write the part set in the 14th century. "You can be more detached from historical characters. Too much of me tended to creep in with the modern characters."

She found more time to write when her two sons went to school, but still could not find a publisher. "People said when are you going to give up?' I said I'll stop when I'm not getting any better.'"

And now, at 45, she has achieved success, after her book was chosen for publication by Macmillan New Writing, an initiative set up in response to the difficulties unpublished writers face trying to get their book off a snowed-under agent's or publisher's slush pile. "The vision was to have a publishing house that just existed to provide a service to new writers," Alis explains. They are only interested in books that require minimal editing, offer no advance, and make certain other conditions. Many people in publishing thought it wouldn't work, but they appear to have been proved wrong. Macmillan publishes about one in 1,000 of the e-mailed manuscripts it receives each month.

Alis is currently writing a book about someone who is profoundly deaf, and trying to see the world through her eyes. She is happy that she now has the time to write, usually four days a week, as she now works one day as a speech and language consultant.

"If you're inventing a world, it's so hard to find the mental space to get back into it, especially at the beginning of a book, when it's quite fragile. Now I can live there a lot of the time."

Testament is published by Macmillan at £14.99.