It would seem to me to be a stretch of the credible to consider that a ground floor chimney breast without further openings, extending upwards to carry the flue through a first-floor room to the chimney stack outside, would require a CO detector to be fitted in that room.

I can see two possible interpretations - the upper room has a flue passing through it, or the upper room (defined by the inner surfaces of its walls) is just adjacent to a flue which embedded in a wall (possibly between two upstairs rooms).  A steel tube type flue passing through a upper room would count though.

That said, I remember my parent's next door complaining about heat and smoke appearing in their upstairs bedroom even though they didn't have a fire below - turned out it was from my parent's fireplace and was leaking through the party wall (it was one of those 17th C cottages that was probably built with clay rather than mortar between the stones, but still the possibility exists).

   - Andy.

It would seem to me to be a stretch of the credible to consider that a ground floor chimney breast without further openings, extending upwards to carry the flue through a first-floor room to the chimney stack outside, would require a CO detector to be fitted in that room.

I can see two possible interpretations - the upper room has a flue passing through it, or the upper room (defined by the inner surfaces of its walls) is just adjacent to a flue which embedded in a wall (possibly between two upstairs rooms).  A steel tube type flue passing through a upper room would count though.

That said, I remember my parent's next door complaining about heat and smoke appearing in their upstairs bedroom even though they didn't have a fire below - turned out it was from my parent's fireplace and was leaking through the party wall (it was one of those 17th C cottages that was probably built with clay rather than mortar between the stones, but still the possibility exists).

   - Andy.