Telling your insurance company you have an alarm and getting the paltry discount (insurance companies know better than most how ineffective most domestic alarms are) is usually a really bad idea.
You will find a condition installed in your policy removing cover if the alarm is not set and it is up to you to prove it was set and it went off if there is a break in. For the trivial discount it is better to tell them you have an alarm but do not use it and do not wish it to be considered.
You can get a service visit for GBP15 to GBP20?