In the flashback scene to 1957, "It's Only Make Believe" by Conway Twitty is playing on the radio. That song did not come out until 1958.
The House of Cavendish indeed owns the Dukedom of Devonshire but it is still extant to present day, never has been revoked. Also, Henry also mentioned a Lord Marcel Cavendish but there has never been one in English/British history, not to mention Marcel is more a French name than an English name. Though as the Cavendish family dates back to Anglo Norman times French given names would not have been inappropriate. Normandy is of course in France.
Colin corrects someone, saying he is not a Lord but a viscount. Any member of the peerage is entitled to be addressed as Lord. Whether a baron, viscount, earl, marquess or duke, followed by the place name, surname or combination of the two.
At the top of Norman Sontag's deposition appears the grammatically incorrect sentence "Client has not be named as suspect"