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Just a simple cheapish one really that I can plug into my pc at work to recharge?
These are ok : http://www.decathlon.co.uk/black-vioo-usb-rear-bike-light-id_8224154.html
Quite bright, USB rechargable, but not much side visibility and a bit barrel-shaped.
I'm not sure how secure the mounting is. I haven't lost the one I'm using off my bike, but someone else did - which is how I got it...
I tried a cheap RSP one but it got lots of water in it quickly and stopped working. In the end I got a knog blinder which was about 25 quid but seems pretty good. It's very bright has several flashing modes and the USB plug is built in so you don't need a cable to charge it.
EBC silicone
I've got one of those knog 4 led blinders. Good light but don't beleive the battery lifetime figures.