I would also be most grateful if someone could explain, in simple terms, what the role of DTMA is?
If the military want to move something, the Defence Transport Movements Agency is responsible for making it happen. That's anything to be moved, whether by land, sea or air; whether on military means or through a civilian carrier. This extends as far as the Pickfords van that moves you on posting (DTMA pick up the bill!) They even own their own port for all the shipping moves they conduct. As far as the AT fleet is concerned, DTMA are the strategic tasking authority, meaning that they have the say on what is carried by AT where to and when. In new-fangled weasle-management speak, 2 Gp is a service provider and the pax are DTMA's customers... in an over-arching, proactive, paradigm sort of way.