It was thought the violin had been lost to the sea when Hartley and the rest of his band drowned along with 1,500 others on April 15, 1912.
But in 2006 an unnamed man found the instrument in a leather suitcase in the attic of his mother’s home.
Some experts doubted its authenticity, saying it could not have survived after being submerged in sea water.
But historians, scientists and forensic experts, along with specialist Titanic auctioneers, Henry Aldridge and Son, and Hartley’s biographer, spent seven years examining it and researching the story behind it before declaring they were able to prove its authenticity “beyond reasonable doubt”.
They said Hartley had strapped it to his chest in a leather bag and it had been returned to his fiance, Maria Robinson, before being passed on to The Salvation Army.
From there it was given to a violin teacher, who in turn gave it to the seller’s mother, herself an amateur musician.
It was bought by an British collector of Titanic memorabilia at the sale in Devizes, Wiltshire on Saturday. When the buyer’s premium is added, the total paid was £1.1 million.
The previous record for a single piece of memorabilia from the Titanic was £220,000, which was paid in 2011 for a 32ft plan of the Titanic used in the inquiry into the sinking of the ship.
However the story of Hartley’s band is one of the most enduring of the disaster. Within minutes of the ship striking an iceberg, Hartley was instructed to assemble his musicians to play to maintain calm.
They performed on the boat deck while the passengers lined up for the lifeboats.
When Hartley’s body was recovered, the violin was not listed among the official inventory of items found in his possession. However a newspaper later reported that Hartley was found with it strapped to his body in his leather bag.
The research appears to suggest that it then either simply floated off in the Atlantic or was stolen by somebody involved with handling the bodies.
Whatever its initial fate, a transcript of a telegram dated July 19, 1912 in the diary of Miss Robinson, to the Provincial Secretary of Nova Scotia, reveals it was eventually eturned to her.
She states: “I would be most grateful if you could convey my heartfelt thanks to all who have made possible the return of my late fiance’s violin.”
She had given the instrument to Hartley in 1910 to mark their engagement and had it engraved accordingly.
Miss Robinson, who never married, kept the violin in the leather case as a shrine to her late fiancé until she died in 1939.
Her sister, Margaret then passed it on to Bridlington Salvation Army and told its leader, Major Renwick, about the instrument's association with Titanic.
The research shows Major Renwick in turn gave the valise to one of his members, a local music and violin teacher.
In the early 1940s, the seller’s mother was a member of the Womens’ Auxiliary Air Force stationed at Bridlington. She met the music teacher who later dispatched the valise and violin to her.
A covering letter which has been found stated: “Major Renwick thought I would be best placed to make use of the violin but I found it virtually unplayable, doubtless due to its eventful life.”
It remained there until its discovery seven years ago. It was then taken to the Government’s Forensic Science Service in Chepstow which concluded the corrosion deposits on it were "considered compatible with immersion in sea water".
An silver expert on the council for the Gemological Association of Great Britain studied the plate on the base of the violin and confirmed the plate was an original fixture on the violin and the engraving was contemporary with the hallmarks on the panel that were made in 1910.
The most expensive violin ever sold was a 1721 Stradivarius called the ‘Lady Blunt’ fetched £9.8 million when it was sold in 2011. All the proceeds went to victims of the Japanese tsunami.