When the children are stocking Polly's bolt-hole at Alconleigh the food cans are empty as revealed by their behavior and sound when handled.
The interior shot of the the Duc's residence reveals an infra-red intruder detector: a little out of place for the 1930s setting.
Although the action clearly takes place over several years the children and adolescents at Alconleigh do not look or behave any older over the course of the drama.