Teribus, how are you sure that there is no such thing as a "Field General Court Martial? If that's so, then that would tend to discredit the information furnished by An Pluiméir Ceolmhar. Information that uses such an uninformed term, must certainly be of limited credibility itself.

The UK National Archives makes extensive use of the term "Field General Court Martial" in a section that provides records of courts martial.

http://www.nationalarchives.gov.uk/help-with-your-research/research-guides/courts-martial-desertion-british-army-17th-20th-centuries/

That being the case, it tends to turn a tables just a tad, doesn't it?

-Joe-