I can't recall the exact key, but the melody that goes: "E D C G", then "G D E C", etc.
Is this chime based on a pre-existing tune? How long as it been used?
They represent the quarter of the hour that has gone by. They're called the Westminster Chimes, Westminster Quarters, or Cambridge Quarters.
EDIT: Here is a copy of the piece that people think the chimes originated from. "I know that my redeemer liveth" It's the 5-6 bar of the piece, the melody line.