Hi Jack,
Take a number of long exposure darks, at least as long as your longest exposure and with the camera at the same temperature. (I normally try to take 20, using cloudy nights)
Median combine them and subtract the master bias (offset) (ISIS for example can do all this for you) The result is the thermal contribution from the camera, which can be scaled to correct any length exposure
This page describes the master images needed for ISIS (In French but Google translates well enough)
http://www.astrosurf.com/buil/isis/guide_lhires/master/calib.htm
and this page (in English) describes how to generate generate them in ISIS
http://www.astrosurf.com/buil/isis/guide_lhires/tuto1_en.htm
see section 3 – spectral calibration
Cheers
Robin