AAIB bulletin has now been released.
Air Accidents Investigation: S5/2013 - Boeing 787, ET-AOP
"A fire event occurred on a parked, unoccupied and electrically un-powered Boeing 787 aircraft at London Heathrow Airport. Subsequent examination of the fire-affected area has focussed on the Emergency Locator Transmitter (ELT). Two Safety Recommendations have been made."