As they came closer, friendly aircraft would transmit a signal that showed up on the screen as a small extra downward spike – but only if they weren’t all bunched together, and only if the receiver was working properly. There was no other way of telling except, sometimes, by the tightness of the formations and the distance between the groups.

The tension, as they waited, was almost unbearable.

At last, they saw it. That extra little spike. ‘Friend,’ they shouted, simultaneously. Everyone in the bunker cheered, clapping Monty on the shoulder. The system was working again. These were RAF bombers returning to base in a group. She prayed Mark was among them. But that was only about thirty planes out of what they’d calculated to be three hundred who’d flown out.

Over the next hour or so they counted them back, reporting to Stanmore and keeping their own score on a blackboard. The numbers crept up but the formations were all over the place, twenty-plus here and thirty-plus there. It was no doubt they’d had a rough time. Aircraft returning from a raid would often arrive in ragged bunches of sometimes only twenty, or fewer. Even in ones or twos. You could read it from the screen, like a picture in the sky, as you worked your way along the length of the trace, plotting every blip, then back to the beginning again. Plot, range, height, plot, range, height. All of them friendly.

A lone aircraft appeared at forty miles out, very low and coming in very slowly. She tracked it carefully: twenty miles, fifteen miles, ten, then just five. The message went to Stanmore, and she imagined telephones ringing at Sutton Heath, calling code red for the fire and ambulance crews. She prayed that it managed to land safely.

They estimated that at least five planes were still missing. She tried to calm herself by calculating the odds: five out of three hundred had failed to return, less than 2 per cent. Or, put another way, there was a 98 per cent chance that Mark had come home safely. She moved from the screen to the plotting table, but there was nothing to plot, so she continued watching over the shoulders of her fellow WAAFs.

‘Stragglers!’ The shout made them all jump. ‘One, perhaps two, coming in low.’ Everyone ran to look. The blip for one of the planes came closer and closer before passing right overhead. One of them at least was probably safe. The other suddenly disappeared about ten miles out. The message went out: someone called air-sea rescue, giving map references of the last sighting. Ten minutes later a blip appeared, most likely the spotter plane. Kath prayed that the men had managed to get out safely before they crash-landed.

Now only three planes were still unaccounted for.

Then, suddenly, a single plane at sixty miles, then thirty, flying low. Another straggler? They tracked it minute by minute. But something was wrong. It began to act strangely, rising, then levelling off before dropping again. It turned and flew away from the coast, then steadied, as though it was moving along the coastline.