Help to identify a film

Late 1960s, early 1970s film, with a British lead male and female actor, setting out to recover a large diamond, possibly in the south of France, on the Mediterranean coast.
Chewing gum is placed on the sensor on the frame of an external door to disable a security alarm.
Final scene is on a plane. The male and female lead are looking at a large diamond, the female lead is placing it against different fingers to decide where best to wear it. A female flight attendant sees the diamond and the male lead tells her it is ‘paste’

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