Somehow, I don’t think that’s right. 
Soldered the pins onto the board and it is now reporting the correct voltage.
Obviously the one that got away during manufacture.
The crystal and D5? So how was it appearing to work at all? (@Amaravati never mentioned an external DS18B20, so I assume D5 not connected made no difference.)
But why/how an apparently correct first reading, then about half what it should be? That I don’t understand.
@Gwil - it possible you are counting the pins clockwise from the “pin 1 corner marker” so you are looking at pins 25 and 24, which makes more sense since ADC1 (batt voltage) is on pin 24 (the 9th pin clockwise).
[edit - better pic with pin numbers]
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Yes, sorry everyone - Paul is correct, the pins were 25 and 24.
Thanks for reporting back @Gwil, good to see there was indeed something not properly connected on the board. It was rather strange to have a first ‘correct’ reading, and subsequent wrong readings…