This might be a red herring, but found a Jno D Martin of Furnivals Inn London Attorney taking on an apprentice in 1797. So could Diggory been articled in the legal profession?
Could not find his birth in Cornwall with a father named Jno.
Certainly an interesting "case", have gone back to family history research after finishing the decorating!! Have found an ancestor (brother of my great grandfather) who went to Australia at 17, after disappearing off the census here. So have achieved something there!!