There's only one direct flight per week (with Kenya Airways, outbound on Fridays, returning on Saturdays). The flight times are 10 hours 45 minutes outbound and 8 hours 55 minutes back.
At other times, you have to travel via Nairobi, which adds to the journey time. For details, check the schedules here (remembering that Kenya is 2 hours ahead of the UK): http://www.kenya-airways.com/home/kqprocess.as px?prc=FS