Source D is useful for an historian studying Haig and the Battle of the Somme because it shows that lots of soldiers were killed just to gain a small amount of land. We can see this in "You mean 'Are we all going to get killed?' yes." This example shows that Haig is willing to let people die only to move slightly closer to Berlin.
Secondly the British thought they would win easily, for example we can show this in "the moment has arrived for us to give Harry Hun a darn good British style thrashing," This shows that the men in the Somme were in good morale and thought they would win very without difficulty.
Also in the TV still they show that the tactics of the British army are very predictable, "make yet another giant effort". This sounds predictable because the words "yet another" makes it sounds like these tactics have been used continually and have become very predictable.
Finally the TV series shows that Haig is miles behind the frontline and that his living conditions are so much better than those of ordinary troops. We see this in the final speech when Blackadder says 'his drinks cabinet'. This shows that Haig is living in luxury while the soldiers barely have a bed.
However it is not that useful because it is a comedy sitcom that was made in 1989. Because it is a comedy sitcom they will most likely exaggerate the truth and make people laugh.
Source E is useful for an historian studying Haig and the Battle of the Somme because it tells us that generals do not fight in the wars, this can be quoted from 'The absence of the General, Sir.' This lets us know that the generals are really in touch with the soldiers on the front line and probably wouldn't know what the conditions are there. Also this could mean that they rely on second hand information.
Also we can see that the uniforms and equipment used is very basic, for example the helmets don't look very protective and simple.
However it is a cartoon and they may exaggerate things in order to entertain people and make them laugh.
Overall I think in some aspects both sources are useful such as in source E the picture showing the equipment of the soldiers and also in source D it says that most people will die, which they did. However they may not be useful because they are made in order to entertain people and make them laugh. So I think both sources are fairly useful.