MarkT thanks.
So the line that reads: Power Supply Voltage Range ±2.5V to ±17V indicates that it must go from negative to positive - presumably in approx equal measure.
Although I have poured over many examples in my quest to understand opamps, I don't yet know how to make the evaluation you make here:
For a 1M impedance it would be a 7mV error, for 1k impedance only 7uV (well below the input offset spec of 50uA)
and I'll make it my next aim.
Would a negative regulator be useful such as the ICL7660SCPAZ found here: www.intersil.com/content/dam/Intersil/documents/fn31/fn3179.pdf
Thanks for your help.