I'm not sure what will come of this thread but I thought I'd let whoever's interested know how I'm sourcing free firewood. For me at the moment it seems a bit of a grey area in terms of potential trespassing etc. I've been locating Invasive Rhododendron stands and turning up with my bow saw when no-ones around to take the usable firewood.
I've been taking it from Forestry Commission land and e-mailing them beforehand to let them know what I'm doing (no response however). They have schemes for getting rid of Rhododendron since it's invasive and reduces their usable land so I assume they want it gone. I may start poisoning the stumps once I've cut them however this is even more of a grey area and poison plugs are expensive!
At any rate you can get a fair amount of firewood and kindling this way and I have been selling it but mostly giving it away.