One of the most invaluable primary sources that I have found in researching the Battle of Britain period are the Operations Record Books of the various squadrons. Thankfully the importance of maintaining these documents was well understood in the midst of the battle, and even to this day the RAF attaches great importance to what is now known as the Form 540 (The astute among you will notice that the ORB shown below from 19 Sqn is a Form 541).
These documents not only provide a narrative of combat actions such as we see here, but they give an insight into the tempo of life on a frontline squadron during the battle. In the above example for instance, both 'A' and 'B' Flt were airborne at 1100 until about 1300 and then again from 1410 till around 1530 (you can see the various individual flight times); three and a half hours airborne for an hour on the ground.
Notice the tick marks annotated within the narrative - another important function of the Operations Record Book was to support claims of enemy aircraft destroyed, probably destroyed and damaged.
It's also incredible to see the estimate of the number of enemy aircraft (E/A) encountered; some 300-400! 'A' and 'B' Flt would usually comprise six aircraft each - it's hard to imagine how it must have felt with a dozen friends at your side as you charged headlong towards several hundred fighters and bombers!