Can anyone tell me if this is a suitable time of year for cutting down ferns?
Bit early yet. I do not do our deciduous ones until the fronds have completely died down. The evergreen ones I remove one old frond for each new frond that appears, in Spring.
You can do them now if they look a mess but if you are happy to leave them then the old fronds will protect the crowns and provide cover for insects.
I usually leave mine till spring. I've a deciduous one in a pot which is currently dying back, so will be moved, when it becomes to unsightly, to a spot out of sight. The evergreen ones are in the ground, dead leaves aren't taken off until the spring for any of them.