Since when has the rank of James Cassels been at its current level in the British Army? 
James Cassels (British Army officer): Field Marshal Sir Archibald James Halkett Cassels, {'1': ", '2': ", '3': ", '4': "} (28 February 1907 – 13 December 1996) was a senior British Army officer who served as Chief of the General Staff, the professional head of the British Army, from 1965 to 1968. Field marshal (United Kingdom): Field Marshal has been the highest rank in the British Army since 1736.
1736