Depends if the A/C was cleaned on arrival or was being done before its move to the departure stand.
Also at LHR some of the galley loading / fuelling is done off stand to reduce the time needed on stand. When these jobs are done is in part dictated by ground staff workload.
These issues could well lead to preparation work going on at 16:00 for a 21:00 departure.
Last edited by boxmover; 13th Jul 2013 at 09:18.