Veritas 8 Alarm sounds wen the system is not in use?

In October 2008 I had a Veritas 8 Alarm system installed by and alarm company which has since ceased trading. About a week ago whilst in the property the alarm suddenly started ringing when the system was not in use. I entered the code and it stopped I reset the system and all was OK that is until yesterday when it happened again and also this morning it has gone off once more. It stops sounding once the code is entered however it seems to me that this is turning out to be a reoccurring problem and I am unable to get the installers to sort it out for the reasoned mentioned above and I also really can not afford the £130 call out fee recently quote by another alarm installers can somebody please suggest a remedy/course of action I can take to rectify the problem of the alarm actuating when it is not set.