During the theft on Wednesday morning, Lord March was struck over the head, reports the Guardian.
Both Lord and Lady March were also tied up during the robbery and only released when a member of staff turned up for work at 6.30am.
Among goods stolen was an antique ring Charles II gave to his mistress and a diamond tiara worth £400,000.
“The intruder broke in after scaling a ladder to an upstairs window,”DI Till Sanderson of Sussex police told the Guardian, “and his activity disturbed Lady March, who went to investigate.
“She disturbed the man, who was alone, and he pushed her and struck Lord March on the head, causing an injury to his ear.
“Lady March was then forced to open a safe and the man helped himself to jewellery. The couple were bound before the robber escaped with the items.”
Both Lord and Lady March are reportedly “devastated” at the loss of the irreplaceable jewellery. Police are appealing for information; a 26 year old man is said to already have been interviewed and bailed.