My ten pence worth say's
Buy a working system that the operators know and will restore the capablity NOW. Then go in for the competitive procurement malarky for the next generation platform/system + 1 (ie jump a generation).
It restores the capability quickly and takes the urgency out of the procurement process which should (hopefully) allow for a more focused procurement competition.
Oh and don't let the MoD contract writing 'experts' anywhere near the process....