According to Threshold, it's 56 days notice for a tenancy of 2 years or more. Furhter details here - [broken link removed].
Given that you're living in an area where long term rentals are hard to find, I'd suggest doing this by the book so you're not giving her any excuses to bad mouth you, especially if the area is rural and people are inclined to know each other. Write to her to give notice. If she asks why, rationally and calmly point out the problems you've encountered. Don't get into a shouting match about what should have been done, what wasn't done, etc, nothing will be served from it in the long term.