The Father Christmas Suit - S2-E11
Revealing mistake: When Elizabeth is driving Richard's car to the church dressed in the father Christmas outfit with Hyacinth you can see that the handbrake light is on while the car is supposedly being driven.
The Father Christmas Suit - S2-E11
Revealing mistake: There are many Christmas cards scattered around the house but you can see in a lot of the shots that they have nothing written in them.
What to Wear When Yachting - S3-E7
Revealing mistake: At the beginning of the episode, when the TV repairman is at the door, you can see that the background behind him is a studio wall - not the front yard we normally see on that side of the house. This effect also appears in many other episodes.
Answer: I have asked myself the same question for a long time. It is Rosina-Queen's fine bone china, pattern "Olde England." It's the china that appears most of the time in the series. The Royal Worchester double-glazed Avignon (for candle light suppers) is really Royal Doulton Avignon, pattern "The Moselle Collection." And the Royal Doulton with the hand painted periwinkles is in reality of course Royal Doulton Colclough, pattern "Braganza."