So, Rod of iron...
Assuming that the Marshalls personnel in question did feel fit and able and ready and willing to do six hours of voluntary overtime at the drop of a hat, and further feel capable of signing a piece of paper 4 hours in that could put them in jail and/or end their careers if they missed something or got it wrong, and then assuming that they had no other plans in their personal lives that they considered more important than doing said overtime, and that Marshalls were willing to pay them to do said 6 hours worth of overtime (none of which assumptions have been shown to be true), why exactly do you think that the engineers concerned should allocate that six hours of overtime to TVOC, and not to whatever project they are currently working on for Marshalls?
Handy hint: "Because the Vulcan and the TVOC are the centre of the universe around which all else must revolve" is not a useful answer, and neither is "Because the TVOC will post a petulant and stroppy press release if we don't!".
I'm sure if - for example - the A400M testbed was to fly a day earlier off the back of their voluntary out-of-hours efforts (no idea if that is what they are working on, but I guess it might be), Marshalls would have much more to gain than they would from signing off some fixes on an air-display aircraft.
If it never occurred to the TVOC during the many years or so available for planning, leading up to their first air show season, that at some point they were going to be requiring a rapid turn-around from Marshalls in order to make an airshow commitment, and that they therefore needed to contract with Marshalls for that level of response, then the people responsible for that fundamental failure of foresight need to be replaced.
If it did occur to them, but they decided that they could get away without paying for that level of response and instead try to browbeat Marshalls into providing it anyway, then that is worse.
Either way, I assume the direct financial impact of missing those last airshow commitments has got to be far greater on its own than the additional cost of contracting Marshalls to provide a more immediate response, thus demonstrating in one easy step the folly of going cheap in this regard. The wider cost for the project may be that this no-show is the last straw for the project's long term viability (assuming that there ever was long-term viability), which makes an extremely poor decision into a catastrophically bad one.