Ok, mystery solved. A bit of internet-searching turned up a .pdf of BA's timetable for 28 October 2007 to 29 March 2008:
TO Nairobi NBO (from LHR)
28-Oct 29-Mar 1234567 LHR 10:05 4 NBO 21:30 BA65 777 0
28-Oct 29-Mar -2 -4-6 - LHR 20:55 4 NBO 08:25 +1 BA89 767 0
Knew I wasn't going mad, and wonder if the BA089 was also in service at the time the OP is talking about in the first post?