The blue flag, tatty, at half-mast, indicates to those watching on the shore that a shoal is present just beyond the sandbank a quarter of a mile out. The green flag is displayed when no shoal can be seen from the helicopters.

It can be difficult to tell blue from green, at certain times of day, even for those without the condition of Daltonism. In milky light, or when a vapour haze descends, the colour of the flag might be blue or green or grey to any but the most acute observer.

Work has begun on developing more robust dyes for the flags. Some have suggested issuing the watchers on the shore with prismatically-enhanced telescopes. Trained cormorants have also been mentioned in a rogue report, although what role they would play remains sketchy.

“These are testing times,” said a brigadier with a great personal investment in shoals. He was supported by those who dwell on the sandbank, in cardboard shelters, whose voices are so rarely heard above the din.

Divers, shellists, and the cadets responsible for the darning of the flags, are due to appear later in the week.