You can buy a copy for around Â£10. Its also bundled with "The battle of the Ancre" showing tanks in battle. I understand that many of the Soldiers who fought in the later stages of the Battle of the Somme would have seen the film of the first day.
My Great Great Uncle was killed on 2 July 1916 and I was hoping to attend the 90th anniversary commemorations on the 1st and visit his war grave in Beumont-Hamel on the 2nd, alas work commitments prevailed.
I saw that documentary (Timewatch?) where they had identified William Holland, the soldier carrying a wounded comrade on his back along the trench towards the camera - one of the most iconic images of the Somme. It really was profundly moving: they'd found his grand-daughter(?) and some family & army photos of Holland showing him all smart, cheerful and dapper - so the contrast with the footage of him looking sweat-soaked, strained and dishevelled was quite powerful, as was the detail they were able to fill in about the casualty and the time & place.
For your info guys there's a very good documentary out called "The Somme from defeat to victory" which is available from DD Home Entertainments priced at about Â£15. It portrays the attack on Thiepval by the Salford Pals, and the subsequent lessons learned. It was filmed near Ipswich last year and they had a WW1 tank based on a Centurion chassis. It's well worth watching, I think it will be on BBC later this year.