The ADVICE is that you should have a CO2 detector in
(a) any room where there's combustion of either gas or solid fuels. [As Bright Spark points out though, the legislation only applies to solid fuels , not to gas appliances - but, having nearly died due to CO2 from a dodgy gas appliance myself, I'd still recommend the use of CO2 detectors where gas is used as a fuel) ; AND
(b) any room that a flue passes through (even if the flue is totally enclosed) ; AND
(c) in every bedroom.
However the only legal requirement is for a CO detector to be present in any room where solid fuel is burnt. (There is no legal requirement for them to be placed in bedrooms or elsewhere in the property - See my first link, above).