It is refreshing to see this family correctly referred to as the Davies family, and not as Llewelyn Davies, within this forum. Thank you, Andrew Birkin.
Andrew already knows that, being descended from a family of Llewellyns and having a mother with Llewelyn as her second forename, I have a thing about the occasional misuse of the name Llewelyn, as did my mother and her mother. In particular, my grandmother was irritated by Sylvia Davies borrowing the name. Andrew has been most helpful to me in private e-mails, especially in guiding me into keeping a sense of proportion over this issue. After reading this post, he may feel that he has only partially succeeded! Laura Duguid is similarly aware of my thoughts and has helpully explained her view from within the Davies family.
While I am happy to concede that it must have been helpful sometimes to refer to the family as the Llewelyn Davieses, in order to distinguish them from all the many other Davieses in the land, and that in more recent times some descendants in the family may have created a compound surname for themselves and their descendants, I am concerned that some people believed, and others may still believe, that Llewelyn Davies was a compound surname for this family in the 19th century. It was not.
From birth certificates I have established, to my own satisfaction anyway, that during the 19th century, at least, the true surname of this admirable family was indeed simply Davies.
For several members of the Davies family, but not all, the name Llewelyn was given as a second forename. That those members subsequently chose, often but not always, to include their second forename in their stated name or signature was, of course, their prerogative. That some of them sometimes omitted their first forename - notably Arthur's father, the Revd. J. Llewelyn Davies - unfortunately, and perhaps inadvertently, seems to have misled others into thinking that he possessed a compound surname. Perhaps this is one way in which compound surnames are created, but I believe they cannot exist officially unless the names appear together (whether hyphenated or not) as the surname of the father on one's birth certificate, or until one's surname is changed by deed poll.
John Llewelyn Davies and Mary Davies formerly Crompton gave their third child the forenames Arthur Llewelyn. Thus when Sylvia du Maurier married Arthur she became (officially) Sylvia Davies, and thereafter she could be referred to variously and conventionally as Mrs Sylvia Davies, Mrs Arthur Davies or (because Arthur sometimes included Llewelyn when stating his name) Mrs Arthur Llewelyn Davies. But, instead, she chose to call herself Sylvia Llewelyn Davies.
There may have been no law against borrowing the name Llewelyn in this way but in doing so Sylvia implied that she had married a man with a double-barrelled surname. Perhaps that was important to her. Upon marrying she had, after all, relinquished the grand sounding name of Sylvia Jocelyn Busson du Maurier! It seems it would have been in character for her to adopt a double-barrelled name if, as Andrew has told me, her father inserted 'du' in his name to create the 'du Maurier' name. Certainly, the Parisian born bachelor lived in London as George du Maurier and married as such.