I have not opened the flour bag, but it is past the best before date, is it still okay to use?

The fact that the bag as not been open will not ensure that the flour is still okay to use.  There is  a window of a few months past the BB when the product is still okay, it is dependent on how it was stored.  If stored cold or frozen the flour can be okay for a long time after the BB.  If you are unsure, try the hot water test.  Put a few tablespoons of flour into boiling water, stir well and sniff.  If product is rancid the smell will come out with the steam.