Hey David,
When will I learn the Logs are always the best place to start 
Looks like a permissions issue with the script
[2021-07-28 12:06:04 +0100] warning/PluginNotificationTask: Notification command for object 'hostname' (PID: 3342667, arguments: '/etc/icinga2/scripts/teams-host-notification.pl' '-4' 'IP '-6' '' '-b' 'username' '-c' 'Test' '-d' '2021-07-28 12:06:04 +0100' '-l' 'hostname' '-n' 'hostname' '-o' 'PING OK - Packet loss = 0%, RTA = 38.04 ms' '-p' 'https://xxxx.webhook.office.com/webhookb2/xxxxx/IncomingWebhook/xxxx' '-s' 'UP' '-t' 'CUSTOM') terminated with exit code 128, output: execvpe(/etc/icinga2/scripts/teams-host-notification.pl) failed: Permission denied
Edited the permissions of the script and now all is working. Thanks for your help!