In the corner of a small, plain room near Salisbury Cathedral – the precise spot must remain a secret for security reasons – conservation expert Chris Woods and archivist Emily Naish are hunched over one of the world’s most precious documents.

Three months after Salisbury’s Magna Carta was the focus of a hammer attack, Woods and Naish are minutely checking the 800-year-old parchment for damage before it is put back on public display on Monday.

As your Guardian correspondent looks on, Woods, director of the National Conservation Service, removes the document from its special protective frame and uses a microscope hooked up to a laptop and his keen naked eye to establish if any lasting harm has been done.

Cathedral archivist Naish, who looks after thousands of documents – some even older than Magna Carta – insisted she was calm. “I don’t get nervous when it comes out of its frame. I know it’s important but it’s just part of the job. You have to be careful. You don’t want to drop it or spill anything on it; you have to treat it as carefully as everything else we look after.”

But the attack at the end of October was a shock. The frame holding Magna Carta sits within another special protective case. It was this outer case that was damaged, causing tiny glass shards and fine dust to fall on to the Magna Carta frame. A 45-year-old man was arrested in connection with the incident.

“Happily, everything worked as it should have done,” said Naish. “It took a long time to make an impact on the outer case and it didn’t shatter.”

A dehumidifier hums in the inspection room as Woods and Naish work, keeping the relative humidity (RH) at about the 40% mark – if it becomes too humid, potentially devastating mould could form.

Facebook Twitter Pinterest Magna Carta under a microscope. Photograph: Simon Czapp/The Guardian

Magna Carta was written on parchment – in this case, sheepskin. “Skin is very reactive to the environment,” said Woods. “If it has taken on some moisture, it will change its shape slightly.”

Magna Carta’s ink – now a rich, dark brown – is stable. But if the parchment expands or shrinks, it can crumble because it does not move in tandem with the skin.

When Woods examined the document outside its frame, it was possible to see gaps and fissures where in years gone by ink has been lost. Also clearly visible was an area of scraping presumably made eight centuries ago by a scribe who scrubbed letters out.

Naish pointed out holes in the corners made by drawing pins and brown stains from where it was once rested on cardboard, vivid reminders that Salisbury’s Magna Carta has not always been so well cared for.

In the 19th century, Naish said, the Wiltshire Magna Carta briefly went missing before being found again in the library. There is also a – perhaps apocryphal – story that one of Naish’s predecessors used to take the document home and store it under her bed for safekeeping. “I think there may be a slight element of truth in that,” said Naish.

Renewed interest in Magna Carta in 2015 – its 800th anniversary year – meant the cathedral was given money from the Heritage Lottery Fund to improve the way it is looked after and displayed.

While they worked, Woods and Naish spoke of their recurring nightmares about losing the documents. But they have fond memories of 2015 when the four surviving copies were brought together at the British Library in London.

“All four were moved to the Houses of Parliament for one day,” said Naish. Each was transported in its own van driven by armed guards with motorcycle outriders. Naish rode with the Salisbury copy, and Woods accompanied Lincoln’s version as they whizzed through the capital. “It was very exciting,” said Naish. As they reached Westminster, by chance or luck, the song Nobody Does It Better from the James Bond movie The Spy Who Loved Me was playing on the stereo of the van in which Naish was travelling. “That was quite a moment,” she said.

At Westminster, a high-ranking official said security for transporting the Magna Cartas had been tighter than that laid on for the Queen, explaining: “You can replace the monarch, but not the Magna Carta.”

The inspection complete, Woods and Naish declared themselves satisfied. The only damage was to two modern paper hinges that help hold Magna Carta to the board it rests on. They were easily replaced.

All being well, the document will be safe for at least another 800 years. “Skin can last an extremely long time,” said Woods. “If mould gets in, it can turn it to dust. If that doesn’t happen, and we’re here to make sure it doesn’t, it should last indefinitely.”