Summary of the turning point of the #BattleOfBritain in primary, contemporary, sources:
The first RAF bombing of Berlin on 26 August 1940 was intended to target factories and airfields but there was 10/10 cloud and only 35 of 86 aircraft report bombing a target in Berlin - and many of those record in the squadron record books that the effect on target could not be observed.
The German assessment of the raid was that it was targeting residential areas and explicitly state that this was the reason Hitler ordered the bombing of London on 7 September.
On 7 September, and unaware of the switch to targeting London, Fighter Command's leadership assessed that if the present rate of attrition carried on for four more weeks it would be difficult to maintain more than a front line defence.
On 12 September Keith Park, commander of the RAF's front line defence, assesses that the change in the Luftwaffe's attack strategy since 5 September means that he is confident in maintaining air superiority over British territory.
Full list of sources:
* https://battle-of-britain-diary.org.uk/1940/08/26/raf-bomb-berlin-for-the-first-time/
* https://battle-of-britain-diary.org.uk/1940/09/07/dowding-has-a-conference-to-discuss-how-to-manage-fighter-commands-pilot-losses/
* https://battle-of-britain-diary.org.uk/1940/09/12/park-issues-report-on-german-air-attacks-on-england-8th-aug-10th-sept/