The reason they had to go was that they trap damp air down by the house wall which makes the front room (G's study) damp and cold. With the ferns gone, the sun can penetrate further (when there is sun!) and the wind will be able to dry the sloping bank and the house wall. The ferns also hid the fact that dead leaves accumulate down at the bottom of the bank, again encouraging dampness.
By lunchtime the bank was clear of big ferns, though there is still a multitude of little ferns that will need weeding out. I also need to sow grass seed on all the bare bits of earth, but the heavy work is done.
I had thought I might do a bit more after lunch, the the heavy grey clouds that had been building up decided to shed their rain, so I'll do some decluttering indoors instead.