Old fern, time to divide?

My fern is about 20 inches at the widest and is many years old.  The fronds are at the "knuckle" stage and they can grow about two feet long.  Is this the time of year to divide itor should I wait until autumn?  I live in Wiltshire about 4 miles north of the M4.  Weather is sunny and warm at the moment.
Thanks.