Posted: 19/02/2015 at 13:56
40 yards of either retaining wall or sheet piling, if it was to achieve anything, would cost a lot. Is the bank actually yours or the farmer's? I suspect you may be worrying too much about stability if it's been the way it is for 40 years. His cows are probably indoors for the winter just now, but no doubt they'll be interested in what you get up to!
Just plant it to make it look nice.