I tried to use my EX1 on mains power yesterday, for the first time, using the BC-U1 ac adaptor/charger. I plugged the adaptor into a mains outlet, selected DC out and fired up. The tally lamp started flashing and I got an immediate warning on the LCD which said "insufficient voltage". Just tried again at home, same thing. No problem charging batteries when set to charge. Any ideas
BTW. DC in reading is 12.3v

Check the settings of your voltage warning in the camera: you can select a voltage level where the camera warns you for low voltage.
I believe default settings should be around 11V so I have no clue why you get the warning at 12.3V.
This might not be the solution; in that case try to test with another U1 if possible.
Otherwise I'd suggest to have your camera looked at...

Good luck!

Hi Walter, thanks for that. I have just come off the phone with Prime Support, They told me to reset the camera, it did the trick. Everything back to normal.