We use Banham and they are great, although I think they only fit/ maintain their own alarms. A monitored alarm is the best, but costly. We have one and have never had to use the service, fortunately. However, a friend has the same system as us and they got broken in to whilst on holiday. Banham were fantastic with them - a guys came over in 5 mins, secured the place until the police arrived, arranged for the back door to be boarded up and made safe, kept my friends informed all through the process. When they got home, Banham had even left out all the forms etc. that they needed to make their insurance claim.
If you just have an alarm, it just rings and people ignore it. No use if you're on holiday and also, Banham will hold keys for your place as part of the service.