Banham are great, superb customer service, engineers arrive as sceduled etc. We have their monitored service with key holding. We've not ever had a break-in, but a friend did when he was on holiday. His experience with Banham was great - a guard came over, secured the flat, waited for the locksmith to arrive, kept my friend updated, sorted out documents for the insurance claim etc.
Not cheap though, but would one want a cheap alarm?