I have a disc-specific version of this Topeak rack. The lower set of horizontal bars not only gets the bag (and corresponding center of gravity) lower, but also leaves the top of the rack clear for a trunk bag, etc. I also have one of Topeak's trunk bags that integrates with the rack and it's a pretty slick setup.

I have a single pannier, the Arkel Commuter. I can't say enough good thinks about Arkel and their products. They are truly top-notch. My bag is over 3 years old and I am a 4-season rider. The bag still looks and works like new. Arkel has a UK link on their website so hopefully you can find their products locally. They also have two bags that convert to backpacks, the Switchback and the Bug. If my bag ever wears out (or I find some disposable income) I'm definitely getting the Bug.