9448 alarm tamper light?

While doing some diy (flooring to stop squeeking boards) the internal alarm went off. Reset and checked the area I was working in - no wiring etc for alarm present so can't have damaged it. When setting my alarm to go out the tamper light comes on and after the exit time limit expires the internal alarm sounds with the tamper light still on. Resetting cures the problem (tamper light goes out) until I go to set the alarm again.

Only defect I can find on checking the wiring circuit is a nick in the outer sheathing but no damage to the actual wires.

At a loss to find the reason for the fault. please advise