except the previous requirement which became, apparently, an ex-Requirement
Nope, the requirement is still extant, it never became an ex-requirement, just the solution to the requirement was "not brought into service". Which leads me to:
He has studiously avoided the reason why MRA4 was cancelled, implying it was simply a savings measure. No one in the Treasury will swallow that one, no matter how hard he tries to protect and spin.
The quoted line at the time was that not bringing the MRA4 into service would save £2B over 10 years ie a savings measure (and also a wet finger in the air); now if you know different..............