I emailed Shipton Mills and asked how long a sack would stay good and how to store, and this is the replay they sent, for anyone else who's interested.
Thank you for your enquiry regarding the shelf life of flour. The short answer is yes - at a rate of 2-3 kilos a week, you will easily get through a sack of flour before its best before date. Wholemeal flour has a shelf life from milling of 6 months because of the high "oil" content in the wheat germ. Beyond six months it will start to deteriorate and eventually go rancid in the same way that nuts do. White flour will be good for 12 months as the oils are taken out with the bran during the milling process which is what gives it the whiter colour. We only mill in small quantities at a time and have a high turnover rate so you will have plenty of time left on the expiry dates when the flour reaches you.
Both can be kept for longer providing you keep them somewhere cool (such as a barn in winter!), and they can even be kept in the freezer which will have no effect on the performance or flavour properties.
Just remember though, if you’re using the flour to make bread, make sure you bring it back up to room temperature or warmer before you start to mix your dough as bread likes everything nice and warm in order to get the fermentation process going. On the other hand if you’re using it for pastry, you can use it straight from the barn or the freezer because pastry is the opposite and likes everything well chilled.
As for storage conditions, we would advise against tightly sealed containers as they prevent the flour from breathing and can cause the flour to “sweat” which will reduce its life and quality. Best thing to do (unless freezing) is to leave it in the sack it comes in and place inside a large crock, clean metal dustbin or a plastic bin with a very loose lid or one with holes in. Just ensure the holes are small enough (or covered with a mesh) to prevent insect or rodent infestation if you're keeping it in the barn.