Thanks for the advice so far.
My point about the side window cabs was that as they were also most of the class that were vacuum fitted, if there was a reason for the vacuum fitting on the side window cab variants that related to them being designed for the Colne Valley, then would a non-window cab, vaccum fitted, version also be likely to turn up on the same branch.
And my understanding is the cab window version was designed for the Colne Valley where steam hauled passenger services tended to run the 20-odd miles Haverhill to Marks Tey rather than the Stour Valley where they ran Cambridge to Colchester where there was a turntable each end. Of course they did turn-up on the latter and other places.
Thanks for the tip about the GERS, I am a longstanding member, thoroughly recommend it and will probably post on the site at some point if someone doesnt beat me to it, but felt posting on here will get any answers to a wider potential audience. Also I am slightly less interested in a more perfect model (which is probably beyond my capabilities) than a plausable back story that I can tell myself!
Can anyone tell me the shed code on the other 2 models (LNER and late BR) please?