As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: AN EMERGENCY WAS DECLARED AND THE AIRCRAFT RETURNED TO LHR DUE TO LOW POTABLE WATER INDICATION AT TOP OF CLIMB. THE AIRCRAFT LANDED SAFELY AT LHR WITHOUT FURTHER INCIDENT. MAINTENANCE FOUND MID POTABLE WATER DRAIN VALVE NOT OPERATING CORRECTLY. REPLACED VALVE PER AMM 38-11-28. ACCOMPLISHED SATISFACTORY OPERATIONAL CHECK.