The P7 or MCE LED torch helmet mounted using velcro straps is great. Self contained unit, less than 200g (at least mine is, about 180g including 'mount'). Mine was £40 including 4 batteries (Just over 4 hours of run time on full beam).
Self contained unit, jolly bright, about an hour on full, or longer on medium (the diablo is a very similar unit, but made with a nicer user interface and probably slightly better made, but still with the rubbish 1 hour runtime). Oh and you can get spare batteries for about £2.50 each or something ridiculous, so when you want you can run it on full for an hour, then just swap over the battery, which you can't do with the diablo or similar. Oh and dead cheap.
Joe