An intruder broke into the house sometime after midnight on Wednesday, January 13.
A diamond tiara c.1820 (valued at £400,000), a diamond reviere necklace from the first half of the 19th century (£200,000) and a distinctive Georgian ring including an intaglio emerald carved with a coronet and the monogram CL (for Louise de Keroualle, mistress of Charles II) were among more than 40 items stolen.
Detective Inspector Till Sanderson said: "Working with Lord and Lady March, we are doing all we can try to trace the stolen treasures, liaising with auction houses, stately homes, dealers and those with specialist knowledge. I am appealing to anyone who may know anything to contact detectives on 101 quoting Operation Forster."
A 26-year-old man from Hampshire has been arrested in relation to the incident and is currently in custody.
