I just worked out that for the last four months we've been underutilizing our water jug. The jug has two sections; the top part is a white retangular box that holds the water filter and sits in the clear plastic jug. You fill the top container, which then filters through to the bottom of the clear jug.
It seems that the top section can fit in two different ways. One allows unfiltered water to flow out a spout and mix with the filtered water when you pour it, and the other does not. As a result, until now I'd been filling the jug "half way", that is, I'd fill the top section and let it all filter through before pouring it. This meant that the jug in the fridge was always at half capacity, as I didn't want unfiltered water mingling with my pure water ;)
It turns out that you can turn the top section about 180° and it fits, but blocks off the spout. This means I can have cold, unfiltered water in the top ready for when we empty the bottom chamber.