Richard wrote:

My journey to work is about 4 miles along an unlit disused railway
line (a rough surface but not extreme off road) and about half a mile
at each end on the road which has street lighting.

As you're going over a rough surface, consider the strength of the
fastener and weight of the light. It's just possible that the heavy
cheap light could work free after day after day of jostling. For this
reason, an LED setup with smaller batteries would probably be better.

--
http://www.unmusic.co.uk Michael Reed -- technology, gender, and geek
culture freelance writer