Yip,hard wired smoke alarm systems do have batterys for back up and they do beep when the battery needs replaced.

They do "beep" to let you know that the battery is low.

You should be able to change that yourself.

Mine beep when the pp3 batteries are low. They were installed when the house was built and are hard wired with batt back-ups.

confess, how many of you did not read the bit
"note it’s not a basic fire alarm but a professionally installed linked unit (Niceic approved). "

professional alarms do not have pp3 batteries, this is a professionally installed Fire alarm, not a battery / mains smoke detector.

The conundrum with some of the cheaper ones is they only have on light for Fault.
the fault can be nearly whatever thing.

The batteries should have been checked / changed on last time.

I would say you should pay for the batteries not the call out, but it depends on the T & C of the contract

Its a possibility, but he should not keep charging you for the service calls. When he came there the first time and you were still having the conundrum when he left he really didn’t do his job. When he came back the second time he had prier knowledge of your system and any problems that you had. There is no way that he should be charging you for a second service call.