So a quick update on this.
@Ackland and I took this offline a couple of weeks back and he sent the Stages SPM1 crank to me as I offered to take a look at it for him.
Long story short, it's working again now although I took a bit of a roundabout way getting there.
@Ackland was given the correct information from the Australian distributor as to which wire went where on the PCB, but I suspect that during the original repair, a small solder bridge between two of the connections stopped the unit from working. As I'd more or less disconnected it all to check the strain gauge resistances while I was working on it and then re-soldered the connections, it must have resolved the issue. My inexperience in using a power meter also threw me a bit - you definitely need the crank on a bike with a fair amount of pedal force to actually register readings on the phone app. It was probably working sooner than I'd thought, but my on-bike testing wasn't good enough.
For the record, correct wiring sequence as shown below; red, green, black and white from the top.
If anyone wants the nerdier details I'm happy to share - I know a lot more about power meters now.
Hopefully
@Ackland manages to find a robust way of reattaching the electronics & housing back to the crank and be back to cranking out the Watts again soon.