... de Longfield - Cicily de LONGCHAMP ... de Longfield

I think I'd like to see additional evidence. It could be right, but this is just another internet genealogy.

http://familytreemaker.genealogy.com/users/g/a/r/Joyce-Garner/WEBSI...

I'd have to double check but I seem to remember that the English name Devereux was originally d'Evreux. Or, maybe it wasn't really but someone claimed it was?

I think Wikipedia says you are right -- this unknown de Longfield should be Cecelia de Longchamp. That makes sense.