He was seen cycling in character as the man behind the Enigma machine, Alan Turing.
Local students got to Twitter to post snaps of the famous actor outside their Library in north London.
The film follows the life and work of the famous WWII mathematician and code-breaker.
Appearing alongside the Sherlock star is Keira Knightley - another period film favourite.
We can't wait - but at least the BBC detective drama SHOULD be back soon!
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