Hello Glwyn,
Thank you for reaching out in Apple Support Communities. With the alarm not sounding off at the set time you had, we suggest to delete and re-add the alarm in the Clock app. This is a step that may help resolve this behavior. Here's how to perform these steps:
How to delete an alarm
- Swipe left over the alarm and tap Delete.
- Tap Edit, tap the Delete button
, then tap Delete.
- Tap Edit, tap the alarm, then tap Delete Alarm.
From there, let's re-add your alarm:
How to set an alarm
1. Open the Clock app, then tap the Alarm tab.
2. Tap the Add button
.
3. Set a time for the alarm. You can also choose one of these options:
-Repeat: Tap to set up a recurring alarm.
-Label: Tap to name your alarm.
-Sound: Tap to pick a sound that plays when the alarm sounds.
-Snooze: Turn on to see a Snooze option when the alarm sounds.
4. Tap Save.
These steps are outlined in this helpful article: How to set and change alarms on your iPhone - Apple Support
Please let us know if that helps.
Kind regards.