Thanks for taking the time to reply guys, some useful tips there that will be plenty useful in future.

As to the solution for my problem, pc88mxer, your little debugging routine picked it up instantly. It turns out that running as root had changed the ownership and group of the output files to root and although I'd chown'd and chgrp'd the main output file back to my normal account, thanks to my own stupidity, I'd completely forgotten about a little logfile that the script also writes to. It's now fixed and everything is working fine.

Once again, thanks to all,
Tom.