15 September 1940. Every fighter squadron was committed in battle, the RAF had no reserves.
Neatly dramatised by Churchill (and the movie), but when Park replied 'there a none' when asked what reserves he had, he was only speaking for 11 Group. Although the RAF were no doubt stretched between 13:45 and 15:45 there were squadrons in 10 and 12 Groups available to call on.