I notice that Evans have the Smart Lunar R1 at 50% off at the moment, 9.99.
I have been using a 5 LED Cat Eye TL-610 and while it gives a good light, mine is loose in the clip and it's size makes it a little difficult to attach without getting in the way of a saddle bag (a very small one that I occasionally use). My mate uses the R1 and I have seen that it is very bright and visible from a long way off. So when I saw that Evans was offering it for half price I picked up one.
The unit has 3 LED: 1 big one and 2 smaller. The light modes allow for the big one to be turned off for group riding. Using readily available 2 AAA batteries, it's already won it's place on my bike.
Oops forgot the link http://www.evanscycles.com/products/smart/lunar-r1-rear-light-ec024286