I'd go for the Maplin kits, they drive the LED's electronically, so the power use is minimal (probably under 1A, so you battery would last 10 hours plus). All those pointless LED conversions M&P and Horrible Gherkin sell just replace the lamp element with a resistor, so you are using power to make the world a fraction warmer as well as slightly brighter and can discharge a bike battery in less time that the RAC could take to arrive.
On a quick fix solution, I got a wind-up head torch from Argos. As well as giving light to work by it can be set to flash. I was thinking that if I did end up on the hard shoulder at night, it'd get left turned on, pointed at the verge a few yards behind the bike, or pointed at the bikes reflectors and flashing away.
Andy