Violinist Min-Jin Kym may have just traded a £1.2m Stradivarius for a mature cheddar and pickle sandwich. Kym was reportedly grabbing a bite at a London branch of Pret a Manger when thieves made off with her 300-year-old violin and two extremely expensive bows. "These items hold enormous sentimental and professional value," her insurer said, making no comment on the quality of the sandwich.

Kym, a South Korean-born violinist who has recently performed with the Philharmonia Orchestra, was reportedly on her way from London to Manchester when she nipped into a Pret a Manger near Euston station at around 8:30pm. She was eating with a friend, according to staff member Hafid Salah, who said the pair were "on computers and iPhones and not looking at their bags", when someone snuck away with Kym's black violin case. "She came up to me at the counter and said: 'Have you seen my bag? Call the police. Can you get the CCTV? You have to do something.'" Salah said. "She was really upset and panicking."

Kym hadn't just lost the tool of her trade; the crook had made off with the most expensive of musical instruments, a violin made by Antonio Stradivarius in 1696. If that weren't enough, the case also contained a bow worth £62,000 by 19th-century French luthier Dominique Peccatte and a, er, £5,000 cheapie by the Bazin school.

British Transport Police have issued a public appeal for the return of the items and Kym's insurer, Lark Insurance Broking Group, are offering a £15,000 reward for information that leads to their recovery. "Although [these goods] are extremely valuable, it would be very difficult to sell them," said detective inspector Andy Rose. Distinctive repair marks and a specially moulded chinstrap mean the violin and bows "will be easily recognised as stolen property".

"It's possible the instrument will be offered for sale within the antique or musical trade," he said, "and we ask anyone who has any knowledge of the violin's whereabouts to come forward so it can be returned to its rightful owner."