Thanks for the reply Covkid, and apologies for my delayed response!
I had planned to use cab drawings of other locomotives built around the same time as the Flatirons, however I have found the drawings I was looking for at the NRM. The cab layout is depicted in great detail in Derby Works Drawings 07-7026 and 07-2027, titled 'Pipe and Rod Arrangement' and 'Pipe and Rod Arrangement End Views' respectively. The search catalogue does not specify that these drawings are of the Flatirons which is why I did not initially find them.