|Presumably the CMOS battery polarity was correct (yeah, silly question I know).|
You could try this:
Turn off, remove domestic power, remove main battery if at all possible. Hold power off-on button down for 20 seconds to discharge the motherboard components. Restart and put settings right, See if a further restart keeps the settings. Bit of a long shot but at least it is easy to try.
Always pop back and let us know the outcome - thanks