Can they be though? If the 345s are on the same production line as the 720s, with no crossover between the two, the first 720s were meant to be in service in March, with enough to displace all the non-PRM 317s and 321s by the end of the year, so there should have been numerous 720s running by now - surely by that notion all the 345s would have had to be completed months ago?
Derby are apparently running 5 lines a the moment (though how interconnected (shared resources) some are is open for debate) and there is cross over - what has happened with the initial 720s and 701s is that they have been slotted in between 345 and 710 production which have been running on 2 lines each. The first 720 car completion was about a year ago
The need for the extra 2 cars for 345 lengthening is now much lower with the crossrail delays.
One of Bombardier's biggest issues is where to store all the completed units pre delivery.
TfL/LO & SWR now have an agreement in place as regards 3rd rail and other shared feature testing
The planned 345 production rate was 11-12cars a week which puts remaining 720 production at ~56 weeks once it gets going properly so displacing about 14-15x 4car EMUs per month so less than 8 months to replace the non PRM units (Assuming all the non Renatus 321s and all 317 are non compliant) but the Stadler Stansted 745s units will help with this too.