Hi thanks for all the comments and interesting discussion. I've been away a few days Caving!
The UK agents 'My fenix' suggested i test the lamp this way. Switch on, set to Mid (150 Lumens) leave on without touching/changing anything and see if it is still alight after 10 hours. They offered to do the same at their end.
So after 9 hours the lamp suddenly changed from Mid to Low and then some time in the next hour changed to ECO setting. After 10 hours it was still alight, but very dim and the red LEDs were flashing to tell me to recharge. The guys at My Fenix, got a very similar result.
So it appears they were correct, it is changing the brightness and/or switching it on and off that is the problem.
I then tried it caving. I never once switched it off or changed the brightness. After 8 hours it went to Low and by 9 hours Eco and and red LEDs flashed. So a bit less in the cave, maybe because it's colder there?
So It does what it says on the box, if you treat it right. But it doesn't stay the same brightness (150 Lumens) for 10 hours.
Also which caver wants a lamp you can't turn off for say a photo and can't turn up brighter to look up an aven?
A partial solution has been suggested here, buy a 3500mAh battery. Which is what I am going to do. It should compensate for the turning on and off etc.
I think the battery I was using was good quality, it is a Fenix one. However, it's capacity, 2600mAh is too low for caving.
The more expensive Fenix lamps have 3500mAh batteries as standard. One of them has the HM70 R ha a 21700 battery which sores 4000mAh.
