In reply to Robert Durran:
I moved to 18650 lithium battery headlights and won't be going back to AA lamps sold by the likes of Petzl and BD. For a similar price, the build quality is much better (typically all metal); the battery life is vastly better; the beam is brighter; more tints are available (I go for warmer tints [3000-4000K) that aren't quite so bright but are more pleasant to look at and better in fog); colour representation (if you go for one with high CRI) is better, which makes the world look nicer; and they often have "moonlight" modes which emit tiny amounts of light sufficient for reading or finding something in a tent but without waking you up or ruining your night vision. They are generally a tad heavier (although barely), but most of the weight is in the larger battery with its concomitant benefits, and they still feel good on your head.
I recommend the Skilhunt H04R, which you can pick up for around 40 quid, or the Sofirn SP40 (cheaper, USB recharging slot on the lamp itself and simpler UI, although a bit heavier). Both of them can be locked either with button presses or by unscrewing the end cap just a tad.
If you want to spend a bit more (well, two or three times more), people speak highly of the Zebralights, partly because they have fancy drivers that keep power output constant, although I actually consider gradually dropping intensity to be favourable to a plateau and hard drop-off anyway since our eyes get accustomed to lower intensity over time. I've never used one though, and my Skilhunt has been faultless.
Post edited at 17:00