Ken,
I think you've got the wrong end of the stick. The MOD will not be buying any FSTA at all! You can find a pretty good explanation here:
Airtanker Website
The deal is a PFI. As such, the aircraft will belong to Airtanker who will provide a service to the RAF over 27 years. Here's the bottom line:
The MoD estimates its through life programme expenditure at £13 billion.
That means that the service will cost about £500m a year for 27 years. This will include all aircraft support and servicing, aircrew training and even the provision of some crews.
I don't know whether that's a good deal or not, but once you look at it from that point of view, it doesn't look too bad. Welcome to the brave new world of defence contracts!
Mind you, at least the treasury won't be able to continually keep reducing funding for AAR once this contract is signed. Once it's on paper, the MOD will have to ringfence that money for 27 years!
.... but I wonder what will happen if we stop needing AAR in 15 years time.
Hope that helps.