OK - All the essential applications have been installed including an upgrade to Agenda Fusion. It all seems to be working fine (touch wood!)
After various tests, I'm convinced everything is now working.
One of the frustrations associated with the recent 'alarms-not-sounding' was the fact that I couldn't use a workiong backup file and restore it to my spare XDA (and therefore have at least ONE functioning device) because the spare XDA had a different ROM version and 'fell-over' half way through the restore process.
Quite by chance, I found that I could download the latest ROM upgrade from the O2 website. The plan was to do this, then flash the ROM on the spare device and then restore a working backup from the main XDA - Are you still with me? :)
A 60Mb download later and I'm ready to flash the ROM! Success!!! I then began to restore a 'good' backup from the other XDA. Success too!
So, we're better off than where I was a week ago when I had one XDA with no alarm function and a spare XDA wiith an outdated ROM. I guess sometimes there no gain when there's no pain :))