BOAC, we do have the report of one 'expert' on conditions along the airway that night, someone who was actually there and not in some chair greatly separated by distance and time.
When passing the INTOL waypoint, they [IB6024, a 340 at FL370] encountered conditions typical of the inter-tropical convergence zone. These conditions were particularly severe 70 NM to 30 NM before the TASIL waypoint. They moved away from the route by about 30 NM to the east to avoid cumulonimbus formations with a significant vertical development, and then returned to the airway in clear skies close to the TASIL waypoint.
The original in French:
En passant le point INTOL, il rencontre des conditions typiques de la zone de convergence intertropicale. Ces conditions sont particulièrement fortes de 70 NM à 30 NM avant le point TASIL. Il s’écarte de sa route d’environ 30 NM vers l’est afin d’éviter des formations de cumulonimbus à fort développement vertical puis la rejoint, en ciel clair, à proximité du point TASIL
IB6024 was 12 minutes behind AF447 (before it deviated, and before AF447 looped in its heading).
For reasons known only to the BEA, the interactive track map that was subsequently released omitted both this flight and the Lufthansa 744 that was 20 minutes ahead, and which deviated to a lesser extent. (The LH track can be traced somewhat by the AMDAR record.)
I will leave it to native Francophones to correct, but I believe turbulence forte is usually translated as severe turbulence.