Hi James,
1) I can see no requirement to install any further fuses if your protection is a 5A fuse or 6A mcb. These OCPD's should provide adequate short circuit protection for the fan & wiring. In other countries these fans are installed on 10/16A circuits without further fusing down.
I am not certain about DP switching being a requirement if you do not have a timer circuit.
2) Extract fans with run-on timer are required to have 3 pole fan isolation switches installed for maintenance purposes.
As regards fusing down, if 5/6A mcb, I would not say so.
