Having a high profile friend should certainly work in Katharine McMahon favour when she launches her latest book The Crimson Rooms with a dramatised reading alongside Rickmansworth-born TV presenter Mary Portas – whose latest show Mary Queen of Charity Shops is currently airing on BBC Two. Not that Katharine hasn’t had a degree of good fortune herself – her last historical novel, The Rose of Sebastopol, was shortlisted for the Richard and Judy book awards, went on to sell more than a quarter of a million copies and the film rights have been optioned by Sally Head, producer of Prime Suspect and Tipping The Velvet. It’s just she feels readers need a proper introduction.

“It’s hard for someone to listen to a book they don’t know,” says Katharine. “I’ve done several of these dramatised readings and they work really well as a shared experience. It brings it all to life, especially when you have friends like Mary and my fellow actors from The Abbey Theatre in St Albans doing the different voices.”

Set in the inter-war years, The Crimson Rooms centres on 30-something trainee lawyer Evelyn Gifford, who is traumatised by the tragic death of her beloved brother James in the trenches and struggling to make headway in a male dominated profession. When Meredith, a mysterious young woman turns up on the doorstep of her family home with, Edmund, a child bearing a striking resemblance to her late brother, in tow, Evelyn embarks on a sequence of events that test her judicial powers to the limit.

Katharine tells me the readings will help audiences to visit the “various rooms in Evelyn’s life, her first trip to the murder scene and the pivotal moment of Meredith’s arrival”, though having read two of Katharine’s previous books (this is her seventh) I couldn’t wait to delve into the book’s highly charged atmosphere by myself.

Katharine’s house, on a leafy street in Oxhey Village is a far cry from the drab environs of the book.

Born in Harrow, Katharine confesses that Evelyn’s cloistered household is modelled on her own memories of maiden aunts living in the area.

“I had three great aunts Gracie, Winnie and Madge, who were all spinsters. They were faint eccentric figures who lived together in Harrow and Wealdstone. The boys in the family all married and moved away but the girls didn’t.

“I vaguely remember childhood visits to these kind of dark houses in a time before there was white paint and double glazing.”

Set in a “a pretty run down villa” in Maida Vale and the countryside around Chesham, Katharine drew on her own experiences of quiet walks and commuting to create the backdrop for the novel.

“I researched the Metropolitan line at the London Transport Museum where they had a 1920s carriage with compartments. Of course back then it was a steam shuttle and you’d have to wait hours for a train.”

A mother of three and former teacher, her two daughters having left home and son Jake now aged 15, Katharine divides her time between writing and mentoring creative writers, while also performing with the Abbey Theatre and working as a magistrate in Hemel and Watford. She says her courtroom experience has helped but she found scant information available on female lawyers.

“It’s been very difficult to get certain information on women in the legal profession. I found some articles but they did little more than name the women involved, there was not much on the work they did. I had nothing to rely on except contemporary male accounts.”

Despite this, Katharine has been able to draw on advice from colleagues and her experiences of the legal process.

“Although there have been many change to the courts since 1924 there’s still that same essence of drama and ritual in the melting pot of courtroom. I know the protocols very well.”

Katharine’s dramatised reading with Mary Portas and friends from the Abbey Theatre takes place at Waterstone’s Watford on Thursday, June 11, with another (minus Mary) at Waterstone’s St Albans on Thursday, June 18. The Crimson Rooms is published by Orion Books price £18.99