Some possibilities gleaned from Worldcat I'd like to throw in the ring:
Campion Towers, by John Louis Beatty and Patricia Beatty
The Grove of Green Holly, by Barbara Willard
The Sealed Knot, by Alfred Lowrie
The Children of the New Forest, by Frederick Marryat
Rosamund Fane, or the Prisoners of St. James's

Juliette, I don't suppose it is The House of Arden by E. Nesbit? Was there any time travel involved?

There was a children's book by Andre Norton that was a fantasy novel. Red Hart Magic altho its actually about 2 foster children who live in "modern" (well, 1960's or 70's when the book was written) times who find the model of an old English Inn (the Inn is the Red Hart) and they find themselves going back and forth in time in their dreams to the time of the English Civil War (I think it was--possibly Henry VIII's, but I really think ECW) and finding themselves as "characters" of that time.

This is bothering me because I love this type of book but can't place it. It sounds like there might have been a reference to Edmund Campion, an important 16th century Jesuit who often brought Communion to secret Catholics in their chapels. Pamela Belle wrote several outstanding novels set around the English Civil War (The Moon in the Water doesn't sound as if it fits your description but did come out around then). I Coriander by Sally Gardner is more recent. Daphne DuMaurier wrote The King's General but again that is an adult book. I will continue to ponder.

Can anyone place this book - read in mid-70's - about a girl from a puritan background who is staying with a wealthy family, I think set just after the English Civil War - she fears they may be catholics as they have a fancy chapel attached to the house - they aren't in the end - there may be a character or characters called Campion, possibly a red cover or something about scarlet in the title?