Accenta 8 outside siren not working?

We recently had some painting work done to the outside of the house and the contractor (bless him) silenced the alarm whilst he carried out the painting behind the alarm box. I suspect he removed the alarm box cover, strapped down the tamper device and carried on working before putting everthing back as it was.
Anyway, the alarm system would not work and there were no "day" and "power" lights lit on the numeric control pad. Suspecting a faulty battery in the control box, I ordered the correct Yuasa type and when it arrived, I fitted it. Instantly back to normal -I thought!
But on testing things out, I find that although an alarm situation fires off the outside strobe light and inside warble alarm, the outside siren does not work. However, when I looked in the outside alarm box to check if the wiring was all in place, the tamper alarm sounded straight away.
What might be wrong and can I sort this out (reset it?) myself please?