yup long time and at common law
in 1705 or thereabouts - the Russian Ambassador was indicted for theft.
The 1707 Diplomatic Privileges Act was passed
and an illumined copy in Russian was offered to the Tsar - Peter the Great possibly, by the British Ambassady in St Petersburg, with his eyes cast down, fixed on the floor.
that good enough ?
Diplomatic immunity ( coz the Ambassador has the same status as the Head of State and they always have immunity ) isnt claimed - the person always has it
and I regret very much to report that the act has been repealed and replaced by an Act incorporating the more recent Vienna convention on Diplomatic Privileges
see:
http://en.wikipedia.org/wiki/Vienna_Convention_on_Diplomatic_Relations