Its much simpler than that ...... my interpretation anyway.
In SDSR, someone in the MoD/RAF came up with a budget for the Tornado fleet going forward, and its not exactly decades ago that this was done, so most 'threat assessment changes' excuses are out of the window.
Here we are 12 months or so later, and 'a hole' in the budget has been found aka 'oops we forgot to include this and that' in the budget.
Tradition would have been that the taxpayer paid more, as if the budget was somehow irrelevant, thereby contributing is some way to wards the £38 Billion MOD overspend.
My guess is that someone may have been told, that they have a budget - work within it.
Thats not brutal financial thinking - its asking people to honour their promises and estimates, and to stop using the Exchequer as if its a bottomless pit of cash for the RAF.