Poundland.
I got a couple of their LED rear lights as emergency back ups and they are at least as bright as the 'proper' branded rear light I've got and have about 6 different light settings.
In fact I put one on the bike as a stopgap due to failing batteries and I've left it on it's that good - it's been on most of the winter with no problems. It's been covered with mud, been out in rain that has soaked me to the skin, in cold that has chilled me to the bone and it just keeps working.
The only thing I've had to do is change the batteries the other week.
So two lights and a pack of 8 alkaline batteries = £3.00
http://www.poundland.co.uk/my-cycle-5-led-back-bike-light