One23 Super Bright 0.5 rear light
The One23 Super Bright 0.5 rear light is just that. By my reckoning (not to mention singed retinas of those behind me), its good to eight hundred and fifty metres in flashing - and the lens profile ensures 180 degree visibility.
The price is the real shocker. Only a few years back this sort of performance would set you back thirty odd quid. Here you'll get change from a tenner at many online retailers - batteries included!