If a Maintenance schedule does not include maintenance for a handheld fire extinguisher it is defective and can not be used (actually it needs to be reported).
CAsA Schedule 5 covers hand held's and states that time limits of the manufacturer must be observed.
Now since tis is data direction from CAsA it must be done.
6.1 The replacement or overhaul of time-lifed components required in an Airworthiness Limitations Section of the aeroplane’s maintenance manual and any special techniques required by the manufacturer or an Airworthiness Directive are required to be complied with. If it is clear from the terms of the manufacturer’s requirement that the manufacturer considers compliance is optional, then that requirement is optional.