Thought I'd just post a little update
I had a test of the Fenix in OFD2 (through swamp creek, lugubrious, midnight chamber and Edward's) around a week and a half ago. I've got to say, for it's small size it packs quite a punch. I had it on medium for spot and flood for the whole trip (~5-6hrs of pottering around) and it didn't run out of battery which was nice (which surprised me being on only one 18650).
In short, it's a good light, Thank you Fenix and UK caving.
