Thanks for the input Darrel and Dave.

It is amazing what you can find in the manual when you know where to look

Darrel, can you please explain how you arrived at the value 117 for the Vref_AD constant? I would have expected it to be 0.6/5*1023 = 122

For my project I was able to use another ADC pin to monitor the supply voltage so all is good (for the time being).

Cheers
Barry