Putting to one side the issue of whether this question should be answered in open forum, we first need a definition of what's meant by "overstretch" of the RAF transport fleet.
My definition of normal day to day ops would be that they are pitched at a level where the op tempo can be sustained indefinitely - say at n sorties*/day per airframe.
The next level up might be a "Surge" flying rate where 1.5n sorties/day could be flown for a given period of time (say 30 days)
Finally, there could be a wartime flying rate of 2n sorties/day sustainable for 7 days.
(all figures made up)
I would have expected that the relevant Flying Order Books/GASOs would contain definitions of flying rates. Flying at a surge rate may be a 'stretch' - but it shouldn't be overstretch (unless you're doing it wrong). The 'sustain' element of the op tempo should take into account all the airframe, human & engineering factors. For example, the aircrew should be rested prior to the next sortie. Crews on separate floors in hotels. The flypro should take account of sleep patterns. On one memorable det, I had 3 weeks of 2 days, 2 nights and a day off. Either all days or all nights but not that.
If 'stretch' is to be truly sustainable and not overstretch then Detcos, Shift bosses & crew captains need to be honest and flag up when the dotted line is in danger of getting crossed. If they don't, then perhaps they should consider alternative employment.
sv
*Sorties or hours.