Gardtec 360/580 "t" message will not turn off?

Gardtec 360/580. We have just moved to a new house and are decorating. We moved one of the PIRs in the kitchen and the tamper message (t) displayed on the control panel and an internal display alarm started beeping on the panel. The detector has been re-fitted onto the wall and we've checked that the cover closed properly but we cannot turn off the beeping and re-set the alarm. We only have one code which we presume is the master code.