There is a wealth of information out there - its just that it is spread widely among many sources!
The Industrial Railway Society is a good place to start - Google their website and ask for help on their Yahoo Group.
Helpful books might be - 'Steam In the Coalfields' by G T Heavyside Publisher David and Charles 1977 which lists the pits and their respective area. For really detailed information on one area look out for 'The Private Railways of County Durham' by Colin Mountford, an IRS publication that has a lot of information about the NCB railways and workshops with loads of pictures [its expensive to buy]. 'Railway Bylines' magazine usually has an article about NCB systems every month.
AFAIK Locos were regularly transferred by road to workshops if not on systems with a direct rail link- for example the mines in West Cumbria had their locos repaired at Walkden in Manchester.
You can see a selection of my NCB steam photos at-
http://mickpope.zenfolio.com/f63169718