ECU - No Comms, No fuel prime, No start

[Griffith500user]

So following a successful first engine start on Saturday with everything behaving as expected and comms to the ECU, come Sunday everything changed. Turn the ignition on with the laptop plugged into the serial port as per the previous day, no fuel pump priming, Code O on the dash pod and the laptop will not connect.

I've tried the following:

• Pressing the inertia switch in the boot (switch yet to be confirmed serviceable)

• Checked 12v power at pin 52 on the ECU connector - all good
• Checked continuity of earths at pins 7,26,50 and 51 on ECU connector, all good
• Cleaned the 19-way under-bonnet electrical connector
• Checked all fuses - all good
• I have bridged out the immobiliser as per Carl Baker's instructions before fitting one of his lovely new alarm systems, plus the car started on Saturday and nothing has changed here

• Located a spare ECU and confirmed this still does not connect

Questions I've yet to bottom out answers to:

• Would a bad inertia switch just cut power to the fuel pump, or fuel pump and ECU?
• I've been told the fuse boards are a potential issue, any thoughts that this would cause my issue?