Plot hole: When Iris Henderson and her two friends arrive at the hotel, Iris orders dinner and asks that it be sent up to "our" room. The waiter arrives when the three are changing. But later, when it's time for bed, Iris says good night to her friends, who leave her alone in the room (as she must be for the sake of subsequent developments).
Continuity mistake: Iris, Gilbert and the other English characters lose their luggage when the bad guys have the back of the train uncoupled. When they arrive in London at the end, Iris and Gilbert are wearing new clothes evidently acquired after their escape. However, they carry nothing when they leave the train. What happened to the clothes they had been wearing before?
Plot hole: Margaret tells her lover Eric that her husband thinks she's "cruising with Mother." Afterwards, when the villains are closing in for the kill, she tells the others, "It's funny: I told my husband when I left him that I wouldn't see him again" - hardly the sort of thing you tell your spouse after telling him a story to cover up your infidelity.