Ok, well I can answer my own question- yes those are the right LEDs. All I did was wiggle the connectors again, and purely out of curiosity plug it back in and not only did LED #1 /501 light up, when I pressed the power button the whole %#*@ thing burst back into life. Typical.
What would be really helpful to this guide, and this image is the closest I can find in the sequence, is a quick summary of exactly where the diagnostic LEDs are on this model (and what they mean, although the latter is easily googleable). I only mention this as I believe my PSU is goosed as no LEDs are illuminated anywhere, however I can only find references to 2009 and 2010 of the similar MYs which evidently have diagnostic LEDs in a different place, hidden under tape, and I suspect numbered differently. So I can't be 100% certain!
