Marrying the mistress

Simon Stockdale spells it out for his son Jack; your grandfather is proposing to leave your grandmother to whom he has been married for forty years and marry a woman with whom he has been having an affair for seven years. What can this mean to Jack in the throes of his first love affair? Or to Simon's wife, Carrie: And what of the mistress - a barrister who has fallen in love with a judge twice her age?