The initial indication is that reinstalling the updates has resolved the issue. The battery life is much better all round regardless of the WiFI frequency. I will monitor the battery for a few days before marking this ticket resolved.
After a bit of experimenting, I have discovered that my XPS 13 Plus roughly triples its power consumption when on 2.4GHz WiFi. It doesn't matter if Bluetooth is on or off, it is not an interference issue with Bluetooth devices. It is also not caused by the WiFi AP. I have tried three different APs and connected to all of them all in both 2.4GHz and 5GHz mode. The result is always the same, battery life drops to a third of what it was seconds earlier when on 5GHz WiFi. Is anyone else experiencing this issue?
XPS 13 Plus 9320 - Huge Power Drain of 2.4GHz WiFi