I fitted my own alarm a couple of weeks ago using a Texecom premier 24 as the control panel and visonic K9-85 PIRs. I had quotes from companies to come in and fit it but they all said they wouldnt give me the engineer code so i would have to pay them to come out and disconnect pirs when i wanted to decorate(at between £80-120 A time). I can understand this if the person having it installed wants it insurance approved but I didn't.
It was pretty easy to fit, the hardest part is running the wires. In all it cost me under £500 and I even have a nice flush mounted chrome keypad instead of the standard plastic jobby, a telephone dialer to text me if it goes off and smoke alarms linked into the system. The quotes I was getting ranged between £1250-1600 and that was for a basic system with no dialer or smoke detectors.
Most of my bits came from
here and the bay.
On a side note I have been told(by a friend) that if you tell your insurance you have an alarm fitted they will give you around 10% off a year but If you get broken into and the alarm wasnt set then they wont cover you for anything. Not worth declaring the alarm for just 10% off a year IMO if its true.