NaB
Thanks. As you say, requirement/budget is set elsewhere. From what you say, it looks like DE&S may have just acted as a post office. You either go back to DEC and say sort out your requirement/budget; or you ignore DEC and prepare a 'clarification paper', which effectively amends the requirement into something that makes sense and doesn't defy the laws of physics.
This sounds rather like the Chinook Mk3 fiasco. A simple requirement to buy X of a known build standard; then someone splits it in two, but doesn't provide the necessary resources to manage two separate programmes. Chinook had the original team doing the top-up buy, and some poor sod given Mk3 to do in his spare time. Very similar. Poor requirements setting and a budget that took no account of development, etc.