If you search on the subject Mark posted a graph showing then the knock sensor works.
Basically on 95RON you are relatively safe as it will drop it into the high det map so the engine wont blow up, but it wont be running anywhere near where it should be and you are robbing yourself of performance for what amounts to a very small amount of money considering the car you are running
Seen it time and time again people claiming that running 95RON is fine, they go on a dyno, have a bit of a