The Cold War secrets behind a butterfly sanctuary

On a windswept hillside in the Mendip Hills, butterflies now drift between wildflowers at Westbury Beacon nature reserve.

It is a peaceful scene today, but this remote site once played a crucial role in Britain's Cold War defences.

In the 1960s, as tensions with the Soviet Union grew, a handful of brave engineers and radar specialists spent their days here, testing missile guidance systems and refining radar technology.

Based between an ancient burial mound and a rusting tin hut, the men stood on top of a nuclear fallout bunker, while aircraft skimmed them at terrifyingly low altitudes.

Among them was engineer Brian Prewer, who remembers the risks involved in this top-secret job.

"You would never be allowed to do this under health and safety today," Mr Prewer said.

"When you're asking a pilot to miss you by 10 feet (3.048m), it's a very skilful job. I often wondered how they never crashed into us - but they never did."

The site's location on the lip of a south-facing escarpment in the middle of Somerset was carefully chosen.

Its open approach meant Canberra bombers, flying from RAF Pershore in Worcestershire, could fly very low without crossing built-up areas, making it the ideal place to test radar with full-size targets.

A mobile radar system was positioned on the Bronze Age burial mound so signals could bounce off the incoming aircraft.

The data recorded here was sent back to the Ministry of Defence, helping to improve missile targeting systems.

"It was cheaper to fly a big radar on top of a plane than to stick a tiny one in a missile and hope for the best," Mr Prewer explained.

"So we were out there, tracking aircraft, making sure our radar could lock onto them properly.

"The goal was to be absolutely certain that if an enemy plane came at us, we could knock it out of the sky before it reached us."

The work was highly classified, and even apprentices sent to assist Mr Prewer and his colleagues were kept in the dark about what they were doing there.

"We told them we were tracking moth migration," Mr Prewer said. "We even made a little sign and put it up by the tin hut. It did the trick."

The team worked in isolation for five years, refining the technology that would shape Britain's air defences.

Getting the data needed was a delicate balance of precision and nerve and Mr Prewer was in constant communication with the pilots.

"I had to tell them if they were off course - too far left, too far right, too high, too low," Mr Prewer explained.

"If they were 30 feet away from us, that was too far and they had to go around again. And we had to make sure the data recorders were running.

"Nothing was automated like it would be today. You had to be there, working, making sure it all happened."

The deafening roar of the aircraft startled nearby livestock, sending cattle into panicked stampedes.

"The farmers were not happy," Mr Prewer recalls. "There was apparently a special budget to compensate them for any cattle that had bolted or worse."

Three Royal Observer Corps personnel, trained to detect and measure the impact of nuclear detonations, accompanied the radar team at the site.

They were housed in a small nuclear fallout bunker, designed to gather data in the event of an attack.

"When a nuclear explosion goes off, whether it's a ground burst or an air burst, it produces an enormous flash of light," Mr Prewer explains.

"The bunker was there to protect the men inside while they measured radiation levels and used photo-sensitive paper to determine the direction of the blast.

"By triangulating with other similar bunkers, they could pinpoint the location of an explosion anywhere in the country."

The radar research contributed to advances in missile guidance and early warning systems, technology that has played a key role in Britain's defence strategy.

"The idea was that if the Russians were flying at us, doing what they're doing in Ukraine now quite frankly, we needed to be able to knock them out of the sky before they got here," Mr Prewer said.

"That's exactly what's happening today. We were trying to make absolutely sure that when you aimed a missile at an attacking aircraft, it would actually hit."

Now in his eighties, Mr Prewer looks back on those years with pride.

"It was not just a job - I really enjoyed being there," he said.

"It was a thundering good job. A bit like the people working at Bletchley Park on Enigma. We were doing something important, something that mattered."

Today, the radar dishes are gone, the bombers no longer thunder overhead, and the tin hut stands abandoned.

It is now a sanctuary for over 30 species of butterflies including Chalk-hill Blue, Wall Brown, Dingy Skipper and Grizzled Skipper, continuing to hide the site's secretive past.

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