Why is Frances obsessed with Tom Fitzwilliam in the novel, Watching You?

Over the course of the novel, Frances becomes increasingly suspicious of Tom Fitzwilliam. Convinced he is the man from their Lake District holiday years prior, Frances assures her daughter that Tom is stalking her. Although Tom has no interest in Frances or her family, Frances is correct that their families encountered one another while on vacation. Frances's often deluded obsession with Tom ultimately helps the cops solve the mystery of Nicola's murder.