I have an alarm but don't want it noted on my insurance, will this affect my policy?

No, you can choose not to mention the intruder alarm on your policy. However, having an alarm fitted can get a reduction in premium.

Also, depending on your contents sum insured there may be minimum security requirements which must be met, e.g. Lever Mortice Deadlocks or alarm. For more significant sums insured, an alarm may be a specified requirement for cover.