Thanks for your prompt reply, I confirm that there appears to be no way to set ISO above 1600 from any menu route. 3200 & 6400 are 'greyed out' (computer style) showing that they are not selectable options.
I was working under low lighting conditions on Rock Group pictures and suddenly the camera seemed to take on a mind of its own. I was unable to remedy nor identify what had happened but was able to continue with it. At the time I thought that maybe I had brushed over a button or something that caused its change of behaviour so felt it better to reset whatever under better lighting later. Unfortunately I am completely confounded ...