As I wrote, it all depends on dates and procedures.
If Jansch published his arrangement at the time the song was recorded, then he has a legitimate claim against Page. If it was done years afterwards, then he can only facepalm.
In any cases, no matter in what year he decided to have his arrangement published, the arrangement is protected as would be an original song (not the recording, which is another kind of right entirely, but the arrangement) against all claims by anyone.
My feeling is that Page's "arrangement" is in a gray area, a period within which Jansch failed to have his arrangement published.