Mrs Lewthwaite used to be Mrs Blackett's nurse, and that of the Amazons when they were young (SA24) & (PM23). She is the mother of the local policeman Sammy Lewthwaite and odd-jobs-man Billy Lewthwaite; Sammy is her elder son (PM23).
She and Billy, and possibly Sammy, live in a cottage not far from Beckfoot (or does Sammy live in a police house?). Cook (Mrs Braithwaite) runs there to get her son Billy Lewthwaite to drive Rattletrap. When Billy is a chauffeur he liked to wear an old yachting cap (PM23,25).