GCHQ insisted on purely symbolic act despite knowing the information was already elsewhere

The prime minister’s deputy national security adviser was punctiliously polite, but firm. We want the computer disks back, he said. We want to do forensics on them. We want to find the trail.

Edward Snowden: 'The people are still powerless, but now they're aware' Read more

It had been almost five weeks since the Guardian’s publication of the Snowden files caused a sensation, exposing the secret global network of surveillance that allowed the UK and the US to spy on friends and enemies alike.

Earlier, in one morning meeting, the cabinet secretary, Sir Jeremy Heywood, had told us: “The prime minister, the deputy prime minister, the foreign secretary, the home secretary, the attorney general, and others in government, have got a problem with you. We want the material back.”

So, with the threat of legal action hanging over the organisation – and the real prospect of our reporting being closed down – negotiations began. The deputy national security adviser was replaced by two amiable GCHQ officers.

Negotiations were straightforward: “We’ll take the computers away and inspect them,” said GCHQ.

“No, you can’t do that.”

The hard drives used to store documents leaked by Edward Snowden are destroyed in the basement of the Guardian’s London offices Photograph: Guardian

“OK, we’ll take the computers away, inspect them and destroy them.”

“No, you can’t do that.”

“We’ll destroy the hard drives in front of you and take away the remains.”

“No, you can’t do that. How about we destroy the hard drives and memory cards while you watch?”

“You haven’t got the equipment.”

“We’ll buy it.”

“You’ll need a degausser.”

“What’s that?”

“It erases the magnetic field on the hard disks.”

“We’ll buy one on eBay.”

“They cost £25k.”

“Oh, right. We’ll use yours.”

It turns out it’s quite difficult to destroy a MacBook. Even with the degausser, angle-grinders, dremels (drills with high-speed rotating bits), masks, gloves – and a lot of smoke and sparks – it was a couple of hours’ work. All under the gaze of GCHQ operatives in a concrete room in the basement of the Guardian’s Kings Place offices. And we kept the remnants.

By the afternoon of Saturday 20 July, the job was done. A purely symbolic act had been carried out. The government and GCHQ knew, because we had told them, that the material had been taken to the US to be shared with the New York Times. The reporting would go on. The episode hadn’t changed anything.

On the way home, still coughing up dust, the phone rang. It was the PM’s deputy national security adviser. He thanked us for what we’d done. He had one more thing to ask: “Have you got the phone number of the editor of the New York Times?”

Postscript: the Guardian’s reporting of the Snowden files continued, winning a Pulitzer prize in 2014 and UK newspaper of the year. In 2015, many of the computer remains went on display at London’s V&A museum. And the prime minister’s deputy national security adviser has a new job. He is Olly Robbins, the civil servant responsible for the UK’s Brexit negotiations.