‘There it is. Two tails instead of one,’ he shouted. ‘A friendly aircraft.’

Scott was whooping excitedly into the phone, ‘We’ve got it, two tails. Yes, two tails. We’ve got it.’

‘Bloody hell, it works,’ Vic said, trying to maintain his focus on the screen. He could hardly wait to share their success with Johnnie. ‘Get someone to give them a thumbs up as they pass,’ he said to Scott.

‘Will do, boss.’

Someone slapped him on the back so powerfully it almost took his breath away. ‘Congratulations, Mr Mackensie,’ Dr Rowe boomed. ‘Your system has proved itself.’

Vic twisted the distance knob, following the signal moving across the screen as the plane approached and then flew past.

‘Good man,’ Dr Rowe said. ‘There’ll be drinks on the bar for your team tonight, lad.’

Even at the height of this triumph he knew that he must manage expectations, especially as he had the full attention of the most important man right here in the room, hanging on his every word. ‘We still need a reliable way of identifying friendly craft in large groups, or in the midst of enemy planes, sir. That’ll need a bit more work.’ A lot more work, in fact; requiring a compact powered receiver to emit a much stronger pulse.

‘Here she comes again,’ Scott announced. ‘First southbound leg approaching.’

The spike grew in length once more, that precious double tail beaming its vital message right into the fuggy room. In future, when the system was fully operational, the information would be conveyed by telephone to a central operations room at Stanmore being linked up to each new receiving station – two dozen of them, in an arc from Suffolk down to the Kent coast – as they came on line.

Vic still found himself astonished by the immense scale of Watson-Watt’s vision, its ambition and audacity. Even though the new masts were now reaching into the skies all around the south of England, very few knew what they were for, or how vital they could be if war was ever declared.

How proud he felt to be part of this great secret project. His eyes swam as he focused on the blip, or tried to, fiddling both knobs, trying to pick up the frequency. The line wriggled across the screen, but he could find no hint of the spike. Where had it gone? He turned the range dial again, one way, and then the bearing dial, then both together, trying to quell the panic rising in his chest. Still no sign. He resisted giving the equipment a kick – it sometimes worked if a wire had come loose.

‘Confirm transmission power still on.’ He tried to keep his voice firm and confident.

‘Roger, power on.’

The instant Scott returned the phone to its base, it rang again.

‘Treble R.’

A pause. Squawking down the phone.

‘Yes?’

More squawks.

An indrawn breath, followed by a loud expletive.

‘Roger that.’

‘What is it, Mr Scott?’ Dr Rowe asked.