Roland Pulfrew, virtually all of this has been covered before.
1, My contention is, that the TriStar could have been economically extended in service much cheaper than lease FSTA pro rata. Just look at KC135, or RJ! There was no need to bring forward TriStar retirement BUT....Air Tanker were going to go to the wall, and we could not have that, could we?
2, Yes. TriStar had selectable scoop settings.
3, I could tell you the cost, but I will not.
Anyway, blinded by the bright light reflected from the shiny new FSTA vision (mirage?), the RAF hierarchy positively ignored the possibility of getting better VFM (and servicability) from TriStar, and effectively left it to rot 15 years or so years ago. The failure to invest anything at all in TriStar caught the Air Staff out when, even superhuman effort by all the worker Bees could not keep the under-invested fleet up to scratch.
4, Widebody tankers (KC10, TriStar K) are generally characterised by the ability to lift over 250,000lb of fuel.
OAP