I've been very impressed with "bigtank's" efforts on building large scale WWI armour. Incredible work!
Where can you find accurate large scale drawings of the British Heavy Mark Series?
Most of what I've come across over time has been in braille scale, and I suspect that the accuracy suffers when you re-copy it to such a larger size like 120mm/1/16th. I do have some works drawings, but while the dimensions are accurate, the outlines aren't.
I would try to get hold of david Fletcher via email and ask him for the plans so he could mail them to you, this isnt free though but its not terribly expensive
P.S. you are the first person ive come across to use WTF as (were to find)
armybook has a very poor record of reliability I ordered several books from them and none came, they usally say they havent recieved them yet from their venodors, whatever that means