[svenrufus]

Don't know the answer in terms of the proportion of wood from different sources, but some of it will very likely have come from overseas, which is tragic if that is the case.
The growth in woodburners can be a real boon to the UK conservation sector if it adds economic viability to woodland management, allowing good conservation work to become self sustaining.
I guess the answer is, that the responsibility lies with the wood buring owners to make sure they get their wood from the most local and sustainable source possible, but I guess the reality is that people on the whole will just go for the cheapest clear felled tropical rainforest woodland that they can lay their hands on.
I notice that many of the hardware stores now sell charcoal made from UK coppiced wood. The woodburner market is an extension of this and to be welcomed if it is managed and promoted well.