Simon Camm's (1999) Gold in the Counties of Cornwall and Devon has a short but interesting section on Poltimore (Bamfylde) mine among others, with lots of useful references. Archaeological surveys of New Florence and Bampfylde appear in Mining History Vol 13.2 available on the interweb at:
http://www.pdmhs.com/BackIssues.asp There's also 2 PhD thesis you migh what to seek out. They might be hard to get hold of but possibly in a local history library in your area
Rottenbury, F J 1974 Geology, Mineralogy and Mining History in the Metalliferous Mining Areas of Exmoor (Univ Leeds)
and
Dixon, D 1983 Mining and the Community of North Molton, South Molton, Molland and Twitchen, Devonshire (Uni Southampton)
Can't understand 'ow I got the sack boay, I burnt twice s'much coal as they other stokers