Thanks. It's possible just to default the engineer code on the Premier alarms.
According to the manual, if anti-code reset is enabled it's also possible to reset tamper alarms using those codes too, which should be a workaround.
In any event, if I wait until there's another fault requiring an engineering reset, I have nothing to lose anyway as the unit will be inoperable anyway.
I'm told by the original installer that they don't lock the NVM - but that remains to be seen!