Due to a bit of sloppy programming on my part the pychef implementation is broken on Mac OS X - one line fault, no idea how it got through, but this will cause failure on any data set run.
The fix is simple: in $XIA2_ROOT/Modules/PyChef/PyChefMain.py add
anywhere near the top - e.g. after "import sys". Thanks to Jim Naismith for pointing out the problem.