PROBLEM FOUND!
The ultimate issue in my system has been located.
Shame it’s not a cake in the manifold.
I popped in gPXE into a machine and got some useful output… The bloody Fortigate isn’t passing option 66! It’s trying to use 0.254 (the gateway) as the TFTP server even though I have calm explained to it that the TFTP server is at 0.90!
What is a guy to do?