Prudence and the Pill

Prudence Hardcastle is on the pill. So is her sister-in-law, but someone has been swapping aspirin for their pills. Is it the teenage niece, the maid, the chauffeur, a lover, Prudence's husband Gerald, or all of the above?

• 
  Fielder Cook
• 
  Ronald Neame
• 
  Ted Sturgis

• 
  Deborah Kerr
  Prudence Hardcastle
• 
  David Niven
  Gerald Hardcastle
• 
  Robert Coote
  Henry Hardcastle
• 
  Irina Demick
  Elizabeth Brett
• 
  Joyce Redman
  Grace Hardcastle
• 
  Judy Geeson
  Geraldine Hardcastle
• 
  Keith Michell
  Dr. Alan Huart
• 
  Edith Evans
  Roberta Bates
• 
  David Dundas
  Tony Bates
• 
  Vickery Turner
  Rose
• 
  Hugh Armstrong
  Ted
• 
  Peter Butterworth
  Chemist
• 
  Moyra Fraser
  Woman in Tea Shop
• 
  Annette Kerr
  Gerald's Secretary
• 
  Harry Towb
  Racetrack Official
• 
  Jonathan Lynn
  Chemist's Assistant
• 
  Ivor Dean
  City Banker in Taxi Cab (uncredited)
• 
  Arthur Hewlett
  Mr. Ferguson (uncredited)
• 
  Michael Hordern
  Dr. Morley (uncredited)
• 
  John Tatham
  Man Sitting in Grandstand at the Races (uncredited)