HELP!! Accenta g3?

hello I have just paid £380 for a replacement pir and a service,
the engineer left at around 4.30 pm.By 7.30pm the alarm sounded even though the alarm wasn't set,
I tried my code but it didn't silence the alarm.

I entered my code several times,then the keypad lights went off.
I still couldent silence the darn thing,

I tried contacting the engineer to no avail,

I tried ringing venom security,but all the lines are constantly buisy.
The alarm has been going off and on all night,.
Can anyone advise me on what to do please,iam at my wits end with it,
please bear in mind iam a oap so don't get too technical