I have now switched to setting up a cron job on my server. I understand that this a better solution for low traffic websites anyway.
After trying many different commands, it appears that the following command works:
php -q -f /home/[user]/public_html/[site url]/wp-cron.php
It doesn't work all the time though, and I am wondering if this is due to the setting in the Group Email Subscription settings page, i.e. that the cron job on my server will only run daily after the time set for daily digests in the settings page. Initially I thought, having disabled the built in WP pseudo cron job, i.e.
that the setting for the daily digest on the Group Email Subscription settings page would make no difference. Items are showing in the digest queue, and running the above cron job would be enough. But because the above cron job doesn't seem to trigger any emails until after the time I have set the digests to be sent on the Group Email Subscription settings page, I now think that this setting does still make a difference and that I need to set the cron job on my server to run 5 minutes after the time set for digests on the Group Email Subscription settings page.
Can anyone illuminate this for me?