Written evidence from the Ministry of Defence to the Defence Select Committee (published 17th Jan 2006) says that the current investment in the Atomic Weapons Establishment (AWE) at Aldermaston is independent of the decision to replace Trident.
According to Annex C of this document:
“This additional investment at AWE is required to sustain the existing warhead stockpile in-service irrespective of decisions on any successor warhead. The investment will sustain core skills and facilities that could also be used in future to develop a successor but no decisions have yet been made either in principle or practice on this issue.” (para. 13)
So I still think/hope that this is a rare case of Mark Thomas being wrong.