We have a couple of hard wired (inoperable) smoke detectors. Can we just replace these? I think these are around 30 years old. Also, do new houses in have to have hard wired smoke detectors? Just curious.

I suspect they haven't been working for some time but yes replace them with domestic smoke alarms however a better solutions is to fit mains powered domestic alarms with battery backup, grade D and interlinked if required. You can get bases that are wired into the mains and the domestic fire alarm is fitted into the base which can be removed if it develops any problems without the need to  to use an electrician to replace the fire alarm.

All new houses require a grade D domestic fire alarms fitted in accordance with the guidance BS 5839-6: 2013.