And there are doubtless a million and one documents from DafT et al around somewhere confirming all sorts of other things to do with what stock was going where and when. But stuff happens...
What exactly was FGW▸ going to operate its services with in the intervening period if they really were supposed to have handed back the 142s before LM▸ got its 172s? The LO sets perhaps, but with all the delays at Bombardier finishing Class 377 orders, even before the exhaust issue arose with the LO 172s, they were never going to hit that target for the release of the 142s and GMITA and all other parties involved would have known that way back in 2009.into this year.
Unfortunately as the new government has removed documents issued by the old government from websites and GMPTA became GMITA with a new website. It means a lot of the documents aren't still online. However, from some printed documents I have I can confirm that the ORIGINAL plan was for Northern to sub-lease the 180s until December 2009, when they would be replaced by the 142s which would be released by the London Overground cascade. This was put back to May 2010 and then December 2010. Only following East Coast's decision not to run Lincoln services was the 180 release put back to May 2011, which allowed FGW to keep the 142s until they got the LM 150s.