So, it turned out that this was entirely self-inflicted.
I had recently added a "Meter Match" device to help calibrate my Centroid fuel quantity senders to the Smiths fuel gauges. These require 12volt power and I simply piggybacked onto a key-on power line that was already going to the sender. Turns out doing so somehow caused power on the small red MSD lead to drop to 7 Volts. Not sure why as the harnesses are all neatly shrinkwrapped but seems that this power lead was shared with the MSD +12 Volt supply. Luckily, I had provisioned to extra hot wires that were unused and moving the Meter Match power supply to one of those resotred +12 Volts at the small red MSD lead.
Thank you everyone and especially Howard Jones who directed me to "put a volt meter on the +12VDC lead AT THE 6AL box. You may have to put in a temporary jumper to do so, note voltage with ign on but not cranking."
Seems I need to brush up on my electrical theory because I'm not entirely clear on why sharing a +12 volt line with a few different components would cause that level of voltage drop but I'm back in business and can worry about other issues now as I am finding we all do endlessly with these cars!