Hi Pierre,
I think you were correct, I also changed my location to avoid any errors that way:
cd /scratch/c.c21087028/Polyposis_Exome_Analysis/bwa/index/indexhumanrefseq_output/
singularity run $CONTAINER index /scratch/c.c21087028/Polyposis_Exome_Analysis/bwa/index/HumanRefSeq/GRCh38_latest_genomic.fa
I'll let you know if it works when its finished, but no errors so far.
Thanks for all your help, Amy