These smoke alarms look like they're as old as the house itself:
I put some other alarms up in 2015 and moved these ones into bedrooms but they were in need of replacing.
There are new minimum standards for fire alarms in Scotland that come into effect on 2021-02-01 that require interlinked smoke alarms in the main living area and hallways/landings as well as a heat alarm in the kitchen.
None of the other alarms are interlinked so I've replaced all of them with mains powered interlinked alarms. There's even a control panel to make the entire house start beeping or identify which alarm is detecting fire when it's not obvious:
I'll automatically get an email if a fire is detected.