Had a girl in it who was going to a tea party or something- but ended up going into the woods to ? look for something or someone? She was told there would be a carriage and/or a cathedral and when she got there, she was in a sort of nature version, under the cathedral she knew well. (Sort of like the Upside Down in stranger things!!) So instead of stone carved gargoyles in the ???crypt?? there were rabbits poking their heads out. And there was a door made of tree roots she possibly couldn't open? When she got in someone was there sleeping, maybe her teacher, sister , brother, mother- someone she was pleased to find or had even been looking for. Maybe someone got married. Maybe there was a giant.
Strong Edwardian clothes feel to it and possibly a vicar in it... also there was an old Gardener/God type figure man giving Mysterious Clues who said "Your tears will turn to [something or other] and your dross to gold". It confused me as I thought it was a sort of "Oooar, young missie" dialect and he meant her DRESS would turn gold. But it didn't.
The copy we had had a blue cover with very tangled intricate fairy-forest type line drawing. The book had fallen in half so I never had read the middle chunk - which could be why I'm so confused now!