kidde 1275h smoke alarm wiring

[BobTBuilder]

During initial installation, from consumer unit right through to the end of the circuit, is there any need for an earth wire at all? as i've got twin and earth that I can use from consumer unit to the first alarm, but i also have three core n/l/e each sheathed in the cable which i'm looking to use to interconnect the other two alarms, rather than use trip+earth (bulkier)
in this instance could i not utilise the sheathed earth as the connector?