Raspberry Pi 3 + TNC-Pi Troubleshooting


Hello all,

Recently purchased the TNC-Pi from John Hansen and have soldered it all up, flashed the SD card and followed the PDF at https://www.tnc-x.com/TNCPi.pdf to the letter. Fabricated cable for Baofeng as per TNC-X website.
However, the radio never seems to Tx up and the TNC seems unresponsive except for the yellow DCD LED flashing. I never have the red LED illuminate or decode any APRS when running xastir
I have 'buzzed' out every connection between component legs to ensure my solder joints are good. Also ensuring continuity from the Pi to the TNC-Pi. I have 5v on the 5v rail and 3.3v at pin 14 of U3.
If I short pins 14 and 6 on U3 the radio TX's - proving the 'output' is fine (transistor etc)
Does anyone know voltages I can check to ensure all is good on the hardware side of things?
If I run 
/pitnc_getparams /dev/serial0 0 
I simply get 'retrying' - even after appending 
in the config.txt file

I've been pulling my hair out over this for quite a few days now.

What am I doing wrong? John suggested to borrow a known working TNC-Pi to ensure my SD card configuration is not at fault and that it is hardware.
I live in West Sussex, UK and work in London so would be able to meet anyone willing to help.
Your help is much appreciated - even if it is simply pointing to resources. 

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