Landlords have to ensure that gas appliances are given an annual safety check by a Corgi-registered assessor. There is no requirement for them to provide CO detectors:
http://ukonline.direct.gov.uk/en/HomeAndCommun ity/BuyingAndSellingYourHome/RentingAHome/DG_4 001394
If a tragedy was to occur, the landlord would not face prosecution (or be liable to pay civil damages) if he could show that he had complied with the law. (i.e. if he did appoint a properly qualified person, who passed the appliances as safe, the landlord should have nothing to worry about. Obviously, if he just got a mate to pretend to be Corgi-registered, he'd be in very deep water). Both civil and criminal action would be brought against the person who passed the equipment as safe, if he was deemed to be wholly or partly responsible for the tragedy. (Similarly, any other person whose actions led to the tragedy could face civil or criminal action. For example, if someone died because the tenant had obstructed a flue, after the inspection had taken place, the tenant could face legal action).
Chris