Board #2 has been finished and is now in place, though it wasn't plain sailing. Although i measured everything up beforehand i forgot to take into account how un-straight all the walls are in my house! The gap between the opposing walls is slightly narrower next to the window than it is a few centimetres away. The board just fits, and indeed is such a tight fit my original plan to put in wall brackets has changed as it doesn't really need them, resting nicely atop too bookcases and wedged in tight!
I've put down some OO track for clearance and testing though i am thinking than plan A which was for HOe is more realistic. I need more track for that though... and a new controller however pretty much everything else i already have. If i did go for OO it would need to be a tiny MPD or engineering siding layout and all of my OO stock (if it still works) would be a bit incongruous. So i think HOe is a more affordable option. The beauty of my new board construction is though that the layout sits atop a foam board layer stapled to the hardboard below. So in theory it would be pretty easy to reuse the board at some stage.