theres a photo a Valiant plus crew on the ground at Nairobi at
138 Squadron
Its said to be on a "Lone Ranger" mission
No date given though this site says the "Lone Rangers" were post-Suez
V-Force Reunion - 138 Squadron 1955 to 1962
While this last site give brief details of the Lone Ranger missions
THE VICKERS VALIANT 199 AND 18 SQUADRONS - Britmodeller.com
Hope this narrows it down
edit
this site has a few more pix of Valiants at Nairobi,
RAF Eastleigh