In a block of dwellings where the heating and hot water are supplied from a communal CHP or boiler there does not need to be a carbon monoxide detector in each invidual dwelling.
However, there does need to be one in the plant room where the CHP unit or boiler is located.
There is only a risk from Carbon Monoxide when fossil fuels are being burnt, with such communal systems this is in the plant room rather than the individual dwellings.