The National Defence Corps (NDC) was formed in 1936. It took men, aged between 45-55, who were either members of the TA or who had previous military experience. Unlike the Home Guard, the NDC were recruited for official duties in Britain (in theory) such as guard duty at camps and airfields. However, when war was imminent, in 1939, a series of laws were passed to cover national service, registration etc. The NDC "contract" was changed so they could be sent overseas, though in what capacity was uncertain at the beginning. This caused quite a few questions to be asked in Parliament as many men had signed up for the NDC under the understanding that they would be used in their local areas.
The NDC was made up Battalions and Groups. Sadly, the NDC isn't very well researched, but they were definitely more official than the home guard.