The supplier(there are companies who work exclusively with supplying vehicles for film shoots) had signed a contract to supply a Norton of a specific type and era. Closing in on the day of shooting the Norton didn't work. Or at least not good enough to be trusted for the shoot. The cost for delaying at shoot is of course massive!
So the solution was to take the most reliable bike around and fit it with a Norton tank. They probably could have showed up with a BSA as well but didn't want to risk a discussion on not living up to the contract and loosing money.
P.s. If you google "Brad Pitt Triumph Bonneville", "Robbie Williams Triumph Bonneville" and "Bruce Willis Triumph Bonneville" you will find them on the same white and gold Bonneville in desert sledge trim. I guess the same company supplied a bike for the different photo shoots.
:D
Just watched the movie.. Pretty good.... Takes a while to get to the bike shot but its a good long shot and you can get a good look at the bike... Really nice bike even with the norton tank ... Thanks again for posting... My wife even like the movie :)
Thanks for the comments folks. I like the bike, even with the wrong tank I think it looks the part. I haven't watched the movie but it gets good reviews, I'll try to get around to it. You can easily find the bike scene just by Googling.