This 35mm screening, which is also being shown in NFT1 on April 30th (full details here), is part of the Sergio Leone season at BFI Southbank. You can find all the details of the season here.
Chicago Reader review:
Sergio Leone's elliptical style and good performances from Rod Steiger and James Coburn combine to produce a vastly entertaining film (1971), also known as A Fistful of Dynamite, about the aftermath of the Mexican revolution. Coburn, a fugitive from the Irish "troubles," and Steiger, a Mexican bandit, team up to rob a bank and unwillingly become the focus of the counterrevolutionary forces. A marvelous sense of detail and spectacular effects--good fun all the way.
Here (and above) is the trailer.