CP2102 using excess current
The answer was the CP2012 was not de-powering (going into suspend mode), and it was drawing about 20mA. I tried bypass capacitors, pull-down on VBUS and D-, pull-up on RSTb with no joy. Sometimes, just sometimes, it powered down; more often with the DC-DC converter regulator for some reason. As I only rarely need UART (debug mainly) I'll put a jumper in place to isolate the UART completely when on battery power. CP21xx devices can be made to run off the USB power only, so powering off the plugged-in cable is an option too.