So, I managed to leave something plugged into the defender and the result was that I ran the battery down. I was able to “jump start” the vehicle and eventually all the electronic warnings sorted themselves out. It’s back to normal now, except for an “SOS Warning”. From research, that indicates I need to replace the SOS battery - but my question is … is it just coincidence that the SOS battery now needs to be replaced? If I do nothing, will it recharge after a while?

 

Probably just coincidence. How old is your Defender?

 

2020. There’s pretty good instruction on this site about changing it. I just didn’t want to go through it if there was a chance it would recharge from normal driving.

 

With the vehicle battery disconnected or completely discharged the telematics unit internal battery will be powering the unit, so no coincidence.
If you can check for error codes you will most likely show a U2109 fault showing telematics battery discharged. Workshop manual suggests you need a minimum of 60 minutes engine running time to recharge the telematics battery followed by clearing the DTC. If this fails then a replacement telematics battery is likely required.