On Saturday I visited a local museum which was screening a contemporary documentary film of the Battle of the Somme film. The Imperial War Museum had digitally remastered the original which it holds in its archives. Many of the images which crop up in programmes on the Somme seem to have been taken from this film. It is in five parts, with 2 covering the build up to the battle and three of the first stages of the battle. The cameramen (who were on the War Office 'strength') had the film back in London for editing within two weeks and the final version was on general release in cinemas before the end of August 1916.
Some scenes were obviously staged for the cameras as propoganda (eg , smiling 'tommies' marching up to the front line), but there are some which capture the mood with more realism. Information on the Somme film can be found on the Imperial War Museum website (the links relating to the film are neat the bottom of the page.)