We’re not quite sure what has prompted this but the co-location of Delta and Virgin Atlantic flights in Terminal 3 has been partially reversed, with Delta’s daily flights to Atlanta and Seattle moved to London Heathrow Terminal 4.
As of today, 29 March 2015, the split of Delta operated flights at London Heathrow is:
London Heathrow Terminal 3
London Heathrow Terminal 4
Philadelphia (launches 8 April 2015)
Delta has since co-located all of its transatlantic flights with Virgin Atlantic in London Heathrow Terminal 3.