dfk41 the council Care Call service they installed another making 5!
As far as I remember, new regulations from recent years require carbon monoxide alarm if you have gas appliances (boiler, heater, hob etc.). These are not the same as smoke alarms, so potentially why you had it installed.
As indicated b @dutchy101 , beeping usually indicates that the battery needs changing + the alarms usually need to be replaced every 10 years (but I think the beeping is more likely a depleted battery, so if you can get it replaced it is likely to cure the beeping). Those batteries are often 9v).