She’d hoped to meet Marcia at lunch, but her friend was nowhere to be seen. When she enquired, one of the other WAAFs explained: ‘She’s probably on duty, or asleep. We’re all on different sites and watches.’

‘Do you ever get a chance to socialise?’ Kath asked. Oh yes, you got thirty-six hours off every four days, they said. And there were dances in the hall every Sunday evening.

‘Plus, if you get the chance to go to one of the USAF dos, go for it,’ one girl said. The others laughed knowingly. ‘The drink and grub they lay on is incredible,’ she added.

‘The music is cracking – real bands, you know, jazz and stuff,’ said another.

‘Need to be good at fending off wandering hands, though,’ warned a third. ‘There aren’t any women on USAF bases, so those fellas are pretty hungry.’

That afternoon the eight ‘new girls’ received their briefing instructions before being given a tour of the site. Bawdsey, having been the centre of research from the very beginning, had more types of equipment than any other, all for different purposes and all known by confusing acronyms such as CHL (which could detect aircraft inland as well as out at sea) and K (for low-flying planes and shipping).

The watch system started with a night shift beginning at eleven and ending at eight in the morning, after which you got the day off and started again in the evening at six. Next day you covered the afternoon from one o’clock till early evening, and the day after that you were on the morning shift till lunchtime. Once again, they were reminded, there was to be no work talk off shift or outside your own site, not even with friends.

At the end of the tour and briefing, Kath was invited to give a brief description of her time at Stanmore, sensing from their questions the admiration and even envy of her fellow recruits. At the end, everyone clapped. ‘Thank you, ACW2 Motts, that was most interesting,’ the leader said. ‘You are very fortunate to have experienced the most vital part of our operation, first-hand.’

She started her first shift that very evening, at the CHL site.

By midnight she had already been on duty for an hour, and her eyes were telling her that she should have taken a proper nap that afternoon. Three large RAF raids had been to Germany earlier in the evening and while many had returned, there were now a number of stragglers on their screens. Some were obviously in difficulty; judging by their heights some were already dangerously low and liable to ditch into the sea. She plotted them continuously, praying that they would make it to the coastline and ready to alert the air-sea rescue instantly should they disappear.

At one-twenty she saw the start of a new track, a faint blip at 115 miles. She found the bearing and pressed the button for the grid reference. It was just off the Dutch coast, she calculated. She switched to the height aerials, pressed more buttons. Height twenty thousand feet. She studied the blip again, estimating forty-plus aircraft, and passed the information through to the plotters, who would send it on to Stanmore. Soon afterwards it was confirmed: identification HOSTILE Track 153. Stomach churning, she took more plots. The range was now a hundred miles, height unchanged, and she could see there were more than sixty of them.