Charles and all,
Here is a reference to the printed article quotted in the first post.
http://www.merfield.com/research/2011/Biochar–the-need-forprecaution-2011-Merfield.pdf
The discussion has been interesting. I noted Monty Don using something black from a big with his potting sometime in later 2016. He didn’t comment and I wondered what it was. Didn’t ask, so don’t know for sure.
I’ll have a look at Northumbria Biochar. and see if it may be better for me.