Therein lies a story.I spend a fair amount of time scanning through books looking for suitable subjects to model.
A Pictorial History of Great Western Architecture is one of my sources and it was inside this tome that the plan was found. However, I could not make out the dimensions, such was the reduction for publishing purposes.
Here the power of the internet comes in. I belong to several groups and I put a plea for information on the GWR e-list. The writer of the book himself, Adrian Vaughan answered my plea and provided all the answers to my questions, even sending an enlargement for my use. Thank you Adrian, I hope my efforts will make the effort worthwhile.