I would go for a failure to comply with article 17: (You could go for article 13, and state that since the detector was removed, the level of detection was not appropriate, but I will go for article 17 anyway)
Maintenance
17.β(1) Where necessary in order to safeguard the safety of relevant persons the responsible person must ensure that the premises and any facilities, equipment and devices provided in respect of the premises under this Order or, subject to paragraph (6), under any other enactment, including any enactment repealed or revoked by this Order, are subject to a suitable system of maintenance and are maintained in an efficient state, in efficient working order and in good repair.
The alarm system, at the point of disconnection of the detector, was clearly not in working order. Meaning that a fire within a room (which someone was smoking in) would not be picked up, (nor would a fire in that entire zone according to the statement of Mr Kurnal) therefore could be allowed to grow undetected in a building where people are sleeping.
32.β(1) It is an offence for any responsible person or any other person mentioned in article 5(3) toβ
(a)fail to comply with any requirement or prohibition imposed by articles 8 to 22 and 38 (fire safety duties) where that failure places one or more relevant persons at risk of death or serious injury in case of fire;.
So I would state that we have a potential offence, as the failure to comply with Article 17(1) did put relevant persons at risk of death or serious injury in case of fire.