Thanks for all the great input. The tamper switch had actually been broken by the Service Company, & they had tried to fix it with tin foil & bluetack!
Luckily as the device was left in Programming Mode, I could reset the Engineer's code, & change the Tamper Smitch.
Then fixed the PIR (which had been disconnected by the Service Company), & fixed 2 Zones that we had to omit for the last 5 years (these were simple problems).
All is good now )
I have one further question that I can't fathom. The Entry/Exit door has a lock with a shunt. When the alarm is in Day mode, the lock USED to give a 2 tone (high, low) indication when it was locked. However, now it gives the high/low indication when the door is UN-Locked, & nothing when the door is locked.
I recall this happened once before, but didn't see what the Service company did to change this back to the 2 tones when locking.
Looking at the programming guide I have "9100/9105 Installation & User Guide", I can't work out what I need to do to reverse the behaviour.
Any tips appreciated, & once again, THANKS FOR THE FANTASTIC SUPPORT.
Martin