The detailed synopsis that, unfortunately, seems now to have disappeared from the BFI website ties in with the clip. I'll come to that shortly.
I watched the film all the way through. It's a mixture of original film from 1916 and staged reconstructions. I jotted down some points that struck me as interesting.
The first 10 mins includes lots of staged shots of "Germans" in their trenches (all wearing full pack . . .). Throughout, the Germans troops are portrayed as automatons and their officers as cruel martinets, but that's only to be expected. The British troops are equipped with box respirators, which, of course, they weren't in July 1916. I also briefly spotted an M1918 Stahlhelm.
There's a sequence showing what must be tunnelling operations, but it doesn't make very clear what is going on. I think you have to have a bit of prior knowledge to realise what is happening. Two men are chatting at the entrance to a tunnel, and the intertitle says, "He'll want an aeroplane for a hearse when this lot goes up."
Later we see a German artillery spotter in a balloon with a camera and telephone. In response to a telephoned request from a British officer, an aircraft appears and shoots down the balloon.
The tone of the film is that the Somme was a gallant effort, not without its costs, but a valiant undertaking and probably worth the effort. It comes close to calling it a victory, but again that is only to be expected.
Now to the tanks. After 1h 13m the tank is described on the intertitle as "A New and Terrible Arm." We then see the sequence shown in the Youtube clip, ending with the Mk V overrunning the German position and the "captured Vickers gun".
It's followed after 1h 20m by a little more footage that is familiar: the two Mk Vs, with grenade nets, surrounded by exploding shells. One of them has a plated-over sponson with a very obviously dummy gun stuck on, the other has a respectable repro 6pdr in a possibly genuine gun shield.
So that's it. We are now in a position to i.d. the tank footage. A problem remains in that some well-known footage that I had come to expect to see in the film isn't there. it starts after about 27 secs here: https://en.wikipedia.org/wiki/File:Tanks_of_WWI.ogg
I had assumed this was post-War footage. The men's helmets look too shiny to be WWI, the uniform looks somehow not quite authentic, as if it's a replica for a film, and the countryside behind the tanks has suspiciously intact telegraph poles and fences, very unlikely on the Western Front. I was sure it would be in The Somme, but it isn't. Anyone know where it comes from?
"Sometimes things that are not true are included in Wikipedia. While at first glance that may appear like a very great problem for Wikipedia, in reality is it not. In fact, it's a good thing." - Wikipedia.