What is the legal minimum for heating houses in multiple occupancy (HMO's)?

my local council has very kindly hit me with the need for a H M O license for my flats. They say that as a term of the licence I have to put a heating system in for the whole of the building (as some of the flats are bedsits it is inpractical to put in seperate systems) Is there a legal minimum of times/temperatures/ and in particular months eg. september to march? for which the heating system needs to operate?