I have read that Bletchley was reading German Air Force Codes in mid-1940
I understand that there was a Luftwaffe unit in France that always used the same 3 letter start groups when setting up their enigma: HIT and LER. They would then send a weather forecast which always started in the same format... these keys allowed the codebreakers to crack the Luftwaffe enigma within a couple of hours (if not sooner) for most days during the War (including the forewarning of the raid on Coventry, but measures could not be taken in case ULTRA was compromised).