I plumped for an Omni V2 in the end, after asking the relevent questions on here.
Thanks to everyone for your very useful replies
. Particular thanks go to Potholer for all the info he gave on lights and batteries/charging :bow:. I feel a little bad that I ended up buying from someone else. Sorry!
The reasons I went with the Custom Duo were that it's simple in operation and does look more professionally made than the Bisun product.
I agree that fitting was very easy. I bought a brand new 5 LED Duo from Wiggle for ?56.00.
The Omni produces enough light to light up large passages and chambers very well.
I haven't done a duration test, just used it at a mix of power settings on the same trip, only flattening the batteries over a couple of uses. My 'feeling' is that duration is less than I hoped but I could be completely wrong about that! Maybe the brand new eneloops I've got haven't had enough cycles to reach their full capacity yet.