I have looked at P&O's website (www.poferries.com), and a pedestrian day trip return costs €8 each way whereas a single without a return costs €35.50.
Are there cheaper options?
I assume that buying the return and then failing to do the return journey will result in a charge to my credit card?
I need to travel from roughly Calais/Dunkerque to roughly Dover/Folkestone, and don't want to take a car.