It should, of course, be noted that, whilst the MOD have possibly the dirtiest, sootiest, highest emitting fleet of transport aircraft in service anywhere in the distant world, if we're talking about the tristar(L1011 for the yanks) and the vc-10 then it should also be noted that they have approximately 2 of each (exaggeration perhaps but the point stands) of which approximately 1 is serviceable at any given time (perhaps less of an exaggeration
..... and they are, in all fairness, shortly to be replaced by a shiny new tanker aircraft (A330
afaik?)....