All smoke alarms now have NEW batteries and pass the button test and the real smoke test.
Had the boiler serviced yesterday and it was suggested that we have a CO alarm in the room where the boiler lives (it's more of a walk-in cupboard). Decided to fit new CO alarms as the ones we have are at least 7 years old and only meant to be good for 5.
Found the reason why one smoke alarm wasn't working:
My excuse is that this was in my wife's office (so nothing to do with me).