Hi Andreas, many thanks! This explains it, as Ubuntu was definetely in warned state when it went critical. I would have thought tgat the delay rule would take into account the notification rule I have set to send out notifications just for critical events and not for warnings.
What I might do then is set APT Updates so that non-critical updates are flagged as OK rather than warning, I assume if I do this the delay rule will work as intened.
As to the “Period Notifications during service problems”, I will give it a go and see how it goes, from its description I miseunderstood it and I thought it was for sending additional notifications rather than suppressing the first one.