Oh dear "Jock McTightSporran" is asking a forum which mainly populated alarm monkeys why their employed.
What goes wrong with your simple house alarm?
Well there will be at least 2 battery's which don't last forever, reed contacts which will eventually fail (open or closed) or cause false alarm.... same for the relays within powered sensors
Checking a bells only system annually as per BS/EN ensures they work correctly (think MOT) if you where a paying subscriber you could have your system in working order rather spending your spare time hunting down a manual for the old tat of an alarm on your wall.
However you pays your money & make your choice,
Power down the "skip fodder" of an alarm system & restart it on mains power only, with all tamper & lids close it made take you code
If not buy something like a veritas 8 & new battery & have a bash at sort it
Or find a local alarm co. (proper ones might only do the work if their maintaining it) & ask for cost of them updating the system