I’ve been trying to find this book forever now and haven’t been able to find anything close.  I’m pretty sure the title has stone in it. The story follows a young boy on an adventure with a girl named Meg, I think there was a hobby horse that could talk. It starts in the boys house where he lives with his sister or step sister who uses him as a housekeeper.  He meets some brownies when sees them at the milk bowl left out for the cat.  There may be an older male character that goes on the adventure as well.  Fairy tales or myths are discovered to be real as the adventure unfolds. I think they have to hide in a well from hunting dogs at one point. The dogs are part of some legendary/mythical hunting party.