I've a while looking for a cheap, yet strong, rear pannier rack for my bike and this ticks both boxes. It takes loads up to 25kg and looks to be made from tubular alloy, as opposed to the rectangular kind with seams that is found on Amazon for around the same price. It's adjustable horizontally as well as vertically so should fit the majority of bikes. And it's got a rear light/reflector plate which was another one of my criterion. Only available in silver.
Down from £19.99. JE James give free delivery on spends over £10 so I just added a cheap, but good, puncture repair kit (http://www.jejamescycles.com/nutrak-p3-puncture-repair-kit-knl1743.html). There are cheaper items to get you over the £10 threshold if you're on a tighter budget.