Firstly, well done for picking up an agent like that - you must have done something really good to impress her! I'd be interested in knowing your secret ;-)
I have spent years reading contemporary literature to familiarise myself with the language of seventeenth-century England and I am also fortunate enough to have access to letters and political speeches the hero of my novel wrote. When in doubt, I turn to good old Shakespeare - but have to be careful, because some of the famous lines of his plays that have become 'well-known sayings' were probably not in wide use until later.
I still don't feel like I'm getting it right sometimes. The funniest thing was a few months ago when I was messaging my friend on Facebook whilst writing a chapter, and instead of replying to her, "Yea, think I'll do that", I wrote, "Aye, I shall take that course". She majorly laughed in my face ... so I must be getting there in some ways.
