Some manuf. say a 6 amp mcb is ok, it's all a bit silly really because a 6 amp 60898 mcb will probably open before a 5 amp fuse but of course we are supposed to comply with their instructions!
Surface mount an un-switched FCU next to the fan housing if zones permit if you want to be super safe.
I now just make sure i always incorporate an fcu with a 3 amp, saves any problems. I am aware that certain manafacturers do not specify a 3 amp fuse in the instuctions...but the majority do.