Probably a silly question, but I just noticed HtM on e23 and I need to ask:
Is there anything much in it about Cornwall? landscape, history etc?
Heirs to Merlin doesn't really go into details about the individual regions. It's got more of a general overview of 13th century English society and history. The only places it details are the major towns/cities (and even then only a paragraph or two) and, of course, the Hermetic covenants. Not to say that it's not a great source. I love it, but if you looking for a detailed description of Cornwall, it isn't here.
There are only a few locations that are covered and then only briefly. There's also one former covenant but it was founded by Criamon and destroyed under mysterious circumstances so little is known about it.