Hi there
Does anybody know if a 70 lux light will be bright enough for riding forest tracks and the light in the dark? ROSE only seem to quote lights in lux rather than lumens so I'm having a hard time converting

Lux is actually more useful than lumens as it measures the light "on the ground". MTB lights are very wasteful as their beam pattern (well lack of pattern) throws a lot of the output light in to the sky - one reason why MTB lights aren't particularly pleasant for other road users. http://gemini-lights.com/explore/lux-and-lumens

RoadCC have a comparison tool that shows the light spread of many lights so you might want to look at that if you haven't already. Peter White cycles has a review of some lights: http://www.peterwhitecycles.com/b&m-hl.asp

I've a B&M Cyo plus which is, I think, 60 Lux. In use it's about as bright as a Hope Vision 2 light since the latter doesn't focus its beam.

I don't think it converts that easily but for reference my Edelux 2 is ~90-100 lux at 18-20 mph flat road pace and is incredibly good at lighting up the road, it's also really good at 10mph off-road where it puts out around 70-80 lux but at climbing speeds it gets a bit dimmer, down to about 35-40 lux based on the output graphs. So a consistent 70 from a battery light should be ok, it's likely that 70 lux is more useful than the 250 lumen Exposure Spark that I can do basic off-road on quite happily. 70 lux as a max for an off-road dyno powered light may get a tad low at 7-8mph though. All depends on the beam spread also, a well-spread 70 lux over a larger area is enough for most things, over a torch-like small round spot with no side-spread it'd be less useful.