Okay, so I'm really confused now!
I have gone back to basics with testing my project by dispensing with all software and circuitry related to the GPIO output, and I just cannot understand the effect I'm seeing:-
If I apply a test voltage of 3v3 directly to the ADCDAC Pi's input the code duly returns the correct reading.
My project requires measurement of a voltage in the range 0-10v, so I use two resistors in series (10k and 20k303) to create a voltage divider. The voltage fed into the ADC is that across the 10K resistor, (ie 3v3 when Vtest = 10v). With the voltage divider in situ the voltage reading reported by the Pi becomes inaccurate as reported before.
Since I can measure the voltage at the input with a multimeter as 3v3 I feel confident to rule out the ADCs input impedance as a factor. These tests are done with just the two resistors and a breadboard.
Am I missing something obvious?