thanks for all your suggestions. unfortunately, my job is in tokyo, with no transfer opportunities, but maybe i can start spening weekends in the inaka
thanks nyororin, i will try out 日本語で結構です more assertively.
i have in fact on occasion told people i was lithuanian (on the assumption that few japanese people study lithuanian) and that made them stop speaking english to me, but clearly that does not work in a professional setting where my job involves writing in english.
i guess what is so frustrating is that even though i am not an english teacher, when japanese people do speak to me in english i make an effort to help them out; correct mistakes in a friendly/subtle way by repeating back to them what they have said using the correct word (thereby giving them the opportunity to not acknowledge that they have made a mistake, but still benefit) and definitely don't switch languages on them (i know how much that stings!)
even if my accent is not great, if they have understood what i said i wish they would help me learn, rather than trying to relegate me to bad-accent status for the rest of my life...