Thanks, the DMM6500 had been on for hours and I’ve run tests in a thermally stable air chamber to eliminate drafts etc and in open air as well as iced water, all showed the same issue.
Finally got an answer from Keithley who stated that basically you cannot just plug a probe into the meter and expect stable readings. It is suggested that I fit the 2001 card, but at over £1000 aears excessive for thee occasional measurement.
However thanks for your input.