Hi Rick,
many thanks for looking into this for me. I did check several times that I had entered all the values correctly, and the differences only tend to turn up when the datasets were in a large directory with lots of other data.
I did run the adwarp on each individual within their directory, rather than using the GUI, so I dont see how this could have caused a problem.
If i see the error again I will let you know. For now I copied each dataset to a new directory, and so far have not had any trouble.
Thanks again
Sarah