It looks like create_IVT_vars.py just needs cf - why not simply do what we do to get cf ( i don’t know how conda environments work in cylc (not well it seems))
pre-script = """
export PATH=/home/n02/n02/dch/cf/bin:$PATH
export PYTHONPATH=/home/n02/n02/dch/cfdm:/home/n02/n02/dch/cf-python:$PYTHONPATH
"""