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 TE AIRCRAFT RETURNED TO LHR DUE TO LAVATORY #3 INDICATES FULL WITH LAVATORIES #1 AND 2 FILLING RAPIDLY. THE AIRCRAFT LANDED SAFELY AT LHR WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED THE #2 LAVATORY TOILET ASSEMBLY PER AMM 38-32-01 AND ACCOMPLISHED A SATISFACTORY OPERATIONAL CHECK WITH ALL LAVATORIES FUNCTIONING AND INDICATING OK.