Forget the monitoring, the police won't turn up.
Get a bells only system with a dialer to let you know of an activation.
£450 for the lot. No monitoring fees; and maintenance if you want it and not if you don't.
I don't think the company who have quoted could it it for that. I specced up the kit and it was around £300 on its own!
For the cost of the maintenance and monitoring I'm not too bothered. It was just the install cost I was concerned about. However, they're getting around £300/400 for the install which in hindsight seems reasonable.