Assuming you had some in the first place and could lay your hands on them in a hurry, the only truthful answer to the question: "What would you save from a burning house?" is: "My love letters".

Diaries are personal too, of course, but there is no more lasting memorial of a relationship than letters written between people who have shared pillow time.

Time only adds to their piquancy, one of the reasons a set of 10 love letters written in the heady summer of 1969 has just sold at Sotheby's for £187,000. Another is that they were written by a 25-year-old Mick Jagger to American-born actress Marsha Hunt, then 23 and later the subject of one of his most celebrated songs, Brown Sugar.

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The Rolling Stones singer, who was still in a relationship with Marianne Faithfull at the time, was in Australia filming Ned Kelly. American-born Hunt was living in Upper Woburn Place in Bloomsbury – we know because that's the address on the envelopes – and was starring in the West End production of Hair. A year-and-a-half after the letters were written, the couple had a daughter, Karis.

The end of the 1960s saw "the ebbing of a crucial, revolutionary era, highly influenced by such artists as The Beatles, The Rolling Stones, James Brown and Bob Dylan", Hunt said after the sale. "Their inner thoughts should not be the property of only their families, but the public at large, to reveal who these influential artists were – not as commercial images, but their private selves." Besides, she added, she needs the money.

It wouldn't happen now, of course: it's hard to imagine a trove of letters surfacing in 40 years to reveal the private, innermost thoughts of today's celebrity couples. We have Twitter and YouTube for that.