You'll need 3810 to get the partition tables, then put on 4020 - this is the way I did it with no such issues.
Right then. I've now downgraded back to 3810 and then upgraded back to 4020. I've also applied the SW 1200 VUCI fix, to ensure no more battery drain occurs.
However, when I tried to recharge my flat battery using my second car it wouldn't start, just clicking that was indicative of a dead battery. I even left the booster cables attached in this fashion for some 35 minutes, but my battery would not take or hold any charge.
I since discovered that this battery is actually wrong for the car, it was a "Unipart" battery with only 64Ah, when it should've been 72Ah at the very least! When I tested the battery with my multimeter, during the boosting, it was in the lower 12 volts. As soon as I took off the booster cables, the voltage dropped to 6 volts, further indicating dead cells in the battery.
So, I went out like a mad man and sourced a Bosch Silver 007 battery with the correct amperage. Well it was 74Ah, but that'll do fine. As soon as I put the new battery in, the car cranked over perfectly.
Now, I can't deduce if it was my possible incorrect SW updates to my RNS 510 that killed the battery, or it was indeed dying.