There are many statutory provisions that deal with building standards and landlord obligations with regard to health and safety (including fire) but I think this question can be answered without dealing with these particular issues in any detail.
As I understand it, the council has suggested a heat sensor system as an appropriate measure to deal with a tenant's enquiry regarding fire safety and his landlord's duties. The result of this is that the management company (which I understand you are secretary of) has, at some point, decided that each of the four flats should have the benefit of a heat sensor system. Presumably this would have been done via consent of all lessees/shareholders.
Whilst all four flats have the benefit of this system, I don't think its reasonable for only the landlord/s of the tenanted properties to pay for its maintenance. However, if there is no requirement or desire for the owner-occupied premises (i.e. the two non-tenanted properties) to continue to have this system installed, there is no need for you to keep it in your properties and contribute to the overall maintenance.
In short, the landlord may, as a result of the request of a tenant, or in order to comply with a mandatory order of the council, install whatever he chooses (so long as it complies with legislation). This can be straighforward smoke alarms or a professionally maintained heat sensor system. However, the owner occupied properties should not be obliged to have the same system and meet a proportionate cost of it (unless it is also required of the freehold management company as a matter of law/regulation).
As the management company has already agreed that each flat should have the system (presumably, upon the vote of the lessees), you may now be bound by a contract to the heat sensor maintenance company, so you may need to check when the terms of maintenance service can be amended.
However, if the council does not require you and the other owner occupier to have the system and you are not in contract with the maintenance company, then I cannot see why you can't have the system terminated in your properties and the landlord/s pay only for the systems installed in the tenanted properties.
The first thing to do would be to check with the council if a heat sensor is required in your properties and whether it is required in any communal areas. If it's not a requirement, then check with the heat sensor company to see what your options are.