Status Update: fixed
So hit the R4 and R5 2k2 resistors, D3 sb diode, and output pins with soldering iron. Worked for awhile and then stopped transmitting again, like before. I then removed these resistors and diode and then bridged over where the 2k2 resistors were and voila, PanelDue is working.
So I tested the 2k2 resistors and they are ok, however SB diode seems to be the culprit in this case.
I'll keep you posted if this arises again.
Many thanks for your help
-Drew