We have a similar problem at the school where I work. A banking, "landscaped" a few years ago, is now slipping into the Tennis Court, bit by bit.
We have had a fresh landscaper in for his opinion, turns out the stone they took out, which new adorns my 20' high X 40-100' rockeries, to keep the rest of the slopes in place, was there to stabalise the bankings.
Now, thanks to one firm! We have to look at either having the banking dug out to a depth of 30 foot, and new large boulders, then smaller boulders, then small boulders put in place, before the banking is re-turfed, OR, have gabian's (?) put in place to a depth of three filled with rubble and faced with cobbles.
Mega bucks!