Paul,
Just spotted your comment about doing your first attempt in a wood. Interestingly enough trying in a wood may be far harder than in a field! I can go down to the local wood, pick up some seemingly dry wood not lying on the floor and attempt to create a coal by drilling. Never seems to work. I can take the wood home to my garden or shed and it works.
A key issue here I think is that under the trees you have a high humidity. Doing it in a dryer atmosphere such as in the open, or in a dry shed is more likely to be successful.