I see this as a continuum from absolutely authentic at one end (eg Chaucerian language only, albeit the spelling tidied up a bit) and anything-is-acceptable 21st century tabloid prose at the other. I think the only difference between us, Peter and Toby, is that the "limits" and "balance" to which you respectively refer are a bit further up the line from me. That's perhaps because I'm interested in the words themselves and how we can use words to provide the necessary atmosphere, showing that the past isn't just the present with an unaccountable scarcity of electronic devices, but was a very different place in attitudes and culture. eg here in Brian's piece, to me there's a subtle difference between relaxation and being at ease, with the former definitely giving an atmosphere of the 20th century.
I also suspect I get pulled out of a story much more easily than you do if I encounter a word which grates on me as being modern -- to me it makes the story a pastiche like one of these Ye Olde Medieval Banquets complete with wenches, Elizabethan costume, roast potatoes and coffee.