Mains smoke alarms have to come of their own circuit/mcb if you want to meet the current regs, 3 core + earth is used as RFLighting mentioned.
Also having them on different circuits can be dangerous and may cause trouble with RCD's as you will have shared the live interconnect between different circuits what will then return to neutral through one of the other circuits neutral, what will trip the RCD's if the circuit of the other smoke alarm is on a diffrent RCD and the interconnect becomes live in a alarm condition.
Also you pose the risk of some one thinking a circuit is safely isolated, when the interconnect and thus the return path through neutral could become live from a another smoke alarm on a active circuit. (effectively, causing a possible borrowed live.) This is amusing you get current flowing from the interconnect terminal to neutral without the permanent live being live.
3 Core + Earth wiring
Brown = Live
Grey (Sleeved blue) = Neutral
Black (Sleeved brown) = Interconnect/Link