Thanks, Kasper!
I tried that but it didn't help, unfortunately. However I've done a bit more experimentation now and it seems as though pressing the caseback into place is what is preventing the alarm from sounding?!! If I detach it but then hold it gently in place, so that the spring is brushing against the piezo plate, the alarm sounds!
So I wondered whether the action of compressing the spring when the caseback is properly clamped down is causing the spring to contact against the metal module cover and perhaps that has the effect of muting the alarm. So I've packed a little bit of BluTack around the spring to prevent it from touch anything other than the piezo plate. But still no alarm sound when the caseback is clamped down. So basically the "looser" the contact between the spring and the piezeo, the more likely the alarm is to sound. Very very odd!
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Cheers,
Dave