From the poster it seems as if the aircraft wore an 'XZ' prefix..... Shots from the movie..
... show that it has an Astazou engine. That rules it out as being neither a British Army machine or G-FILM. These were Artouste powered. The Army Alouettes were XR I think.
Another clue to Mann's non-involvement is that if the film was dated 1970, which is 10 years or so before G-FILM was aquired by them.
The names of the pilot and the aerial camera operator sound rather French to me too ...
The film is set in South America (but shot in Spain). The national mark for Mexico is XA - maybe XZ was a bit of poetic licence by the art department - it wouldn't be the first time.