A Stitch in Time Saves Quite a Lot Apparently

This is the last of a string of ideas I've had since the made any serious attempt to write. As usual, it's AU but this one is based on an idea from Potterlad81's enjoyable Me Without My Tardis. Harry and Hermione are thrown over 50 years back in time from the end of their Third Year with knowledge of what Dumbledore and others have been doing, so they decide to put a stop to it.