To test out the faulty fuel sender unit theory, I did a couple of trips to the servo with a jerry can and poured some 20L of diesel into the tank. Having had the old battery on charge for a few days, I hooked it up and turned on the ignition, to which the car proceeded to have a full on seizure - cluster flashing on and off, relays clicking, alarm beeping etc. Not knowing how old the battery was and the little peep hole staying black even at full charge, I picked up a brand new Century DIN85LHX MF as a replacement.
Before I could hook it up, I discovered the clamp on the negative cable had somehow seized so you couldn't tighten it properly. Took some vice grips and an impact wrench set to full blast to break the stubborn stud in half. Luckily I still had a few spare ones on hand.
Battery hooked up, you could hear the fuel pump priming as you turned the ignition, although there seemed to be all sorts of modules coming back to life and there was all sorts of sounds coming from all directions all at the same time. Gave it a couple of 10-second cranks, on the third one she fired right up! Happy days. Fuel injection system error still lit up the cluster but seemed to idle fine. Took it for a quick lap around the block, didn't really give it beans but didn't feel like it was in any sort of limp mode. Hooked it up to the scanner, still had the old list as well as a few more errors this time around.
It now takes a second or two of cranking but then fires up fine. Cleared all the codes, took it for another quick lap and none of the codes are yet to come back. Great success.
Too soon to jump to conclusions but seeing it run and drive really made my day. Heaps of preventative maintenance to do before she goes into full on daily duties but the beginning looks well promising.