As far as one can gather from the limited information available Air Tanker would expect to have a number of frames available each day for their own use unless they have been given a fairly long period of warning. That is the PFI deal.
If that is the case then why not just lease Global Air Tanker's DC-10's?
The idea that the RAF only has access to limited amount of airframes for Ops is absurd while the other airframes are flying around doing civilian operations.
The VC-10/Tristar fleet are up to their neck in flying AR and Cargo/Pax and so leasing a limited number of airframes to start with which do not tally up to the number of AT we currently have surely is a waste of time. Those limited number of airframes would be in use by the RAF non-stop i should imagine with no 'other use' available by Air Tanker due to the silly numbers being acquired under the leasing agreement.