But when the Fletcher family went for a pre-Christmas treat at the Manor Restaurant in Waddesdon Manor, near Aylesbury, Buckinghamshire, they were disappointed with their meal.

Marilyn Fletcher, 57, sent a letter complaining of slow service and poor food, in a lunch which cost £127 for four.

But she was left astonished by the restaurant's response. Simon Offen, the catering manager, emailed her to say he disputed her version of events after he had "watched and listened with interest to the video recording of her table".

Mrs Fletcher was horrified that the meal had been recorded on CCTV cameras, and said her family found it "extremely disturbing" and felt "outrage at the invasion of our privacy".

In a letter to the National Trust's director-general, Dame Fiona Reynolds, she asked: "Does the National Trust condone recording, watching and listening to private conversations at customers' tables in National Trust restaurants?"