Conclusion: I believe I may have found the answer to fixing the "Invalid Sim" message and achieving the unlock.
Current state: "Invalid Sim"
Run: yellowsn0w - r followed by yellowsn0w -s
Toggle airplane on - 10 secs - off. Reboot. 10 secs later, unlocked.
I will add to this post with my initial findings. Conclusion is written above.
Update 1: Waited 5 mins+. Still shows Invalid sim message.
Update 2: yellowsn0w -r. Invalid Simm message gone. 10 seconds later, full signal.
Will now reboot to see if unlock remains.
Update 3: Before even rebooting, signal lost. Back to 5 dots.
Update 4: Ok,so running -r, signal returns in exactly 25 seconds. 15 secs later "No Service" message followed by 5 dots. Next is toggling airplane mode.
Update 5: Toggled Airplane ON - 10 secs - OFF - Shows "Invalid Sim".
Uupdate 6: -r. signal after 20 secs, followed by "Searching" for 1 sec then 5 dots.
Update 7: -r then -s = "No Service message" for 25 secs. Then 5 dots.
Update 8: -r, wait 25 secs for signal return, then toggle airplan mode on.
Waited 10 secs the airplan off. "Searching" displayed followed by No signal, the 5 dots. "Searching" is good.
Update 9: Toggled airplane mode. "Invalid Sim" displayed. Rebooted iphone. Full signal even longer than 25 seconds. Rebooted again - Signal remains. "Perfect State" achieved.