Thank you - yes it was a permissions issue on the directory for storing php session files.
Here's some more info which may benefit others.
When running php from the command line (as cron jobs do), the php configuration comes from a different php.ini file as it does when php runs from the web, ie under apache control. To remedy the php warnings I had to set the session.save_path (to a writable directory) in the CLI php.ini not in the web php.ini file.
And as you say - it is only a warning not an error - so no real harm done.
However as I'm running the command line as a cron job, errors and warnings sent to stderr which cron emails to the mailto: specified in the crontab file. By eliminating this warning I'm avoiding more pesky emails in my inbox.
Thanks again for your fast response.