Thanks for the kind words guys
I completely agree with Roots. There are literally hundreds of patterns of billhook and while you could use any of them, they have all been developed for particular jobs (with regional varioations of course
). I would avoid an double edged hooks though as they are very dangerous! For coppicing I would go for something with a fair hook on it and a blade of between 9 and 13", based on the ones I've used but that's only me. The older hooks are the better hooks purely because they have good geometry adn are much more user friendly. Personally I prefer no9t to have a turned (ie round) handle as it twists in my hand, but some people like them.
As roots says, try some out if you meet up with anyone who has one and see what suits you
My personal fave is one that I made a few years ago based on a 2nd century AD hook. It doesn't have much hook, so is not so good for coppicing, but for most other things it is brilliant!
You will most likely need to regrind the edge and sharpen it as they get very worn and rounded off in use. To sharpen mine I use wet and dry paper wrapped around and stick. If I'm re-grinding one I tend to use and angle grinder (carefully!) but I have used a 1/2 round file in the past