It is four books, written by a female. Possibly named Diane or Diana? But maybe not.
The first three books are set in the same world and about different characters. One is about a boy who sails on a boat, and there is a scarecrow (or something) that helps him somehow. One of them is about a person travelling with a family in a wagon performing music. He had a magic harp or flute or something. There is one more I can't remember.
The fourth book is about a girl who travels back in time from England (possibly making this a British author?) to the past, and meets the main characters from the other books. They have an adventure. She was staying in a castle that is both in the future and the past.
GRRRAAAAARRGGHH... It was really good, I just can't remember any more about it. Help? Anyone?
But, this is the closest I could find after searching... hope it's what you're looking for:
WINTHROP, ELIZABETH; [i.e., Elizabeth Winthrop Mahony] (1948- )
* * _The Battle for the Castle (Dell Yearling 0-440-40942-X, Dec ’94 [Nov ’94], $3.99, 211pp, tp, cover by Jody A. Lee) [Castle in the Attic] Reprint (Holiday House 1993) young-adult time travel/medieval fantasy novel, sequel to The Castle in the Attic.
* * *The Castle in the Attic (Holiday House 0-8234-0579-6, 1985 [Mar ’86], $12.95, 179pp, hc) [Castle in the Attic] Juvenile fantasy novel of a boy and an enchanted knight. Has a 1985 copyright, but not seen until 1986.
* * _The Castle in the Attic (Bantam Skylark 0-553-15433-8, Sep ’86 [Aug ’86], $2.75, 179pp, pb) [Castle in the Attic] Reprint (Holiday House 1985) juvenile fantasy novel.
* * _The Castle in the Attic (Dell Yearling 0-440-40941-1, Dec ’94 [Nov ’94], $3.99, 179pp, tp, cover by Jody A. Lee) [Castle in the Attic] Reprint (Holiday House 1985) young-adult fantasy.
http://www.sirius-books.com/revjones.html
Seriously, it could take days to track this down. I've been doing Boolean searches to no end... and there is a plethora of websites that come up. No wonder you can't find it.