What is the author's style in Major Pettigrew's Last Stand: A Novel by Helen Simonson?

The language in "Major Pettigrew's Last Stand: A Novel" by Helen Simonson is supposedly based on the average rural British person in modern times. However, the usage is reminiscent of novels from the mid-20th century, which brings about comparisons to characters and storylines used by authors of an earlier era such as Agatha Christie or, more recently, John Mortimer.